Buttermilk Chocolate Sheet Cake

Name: Buttermilk Chocolate Sheet Cake
Prep time: 20 min
Cook time: 22 min
Total time: 102 min
Ingredients: 1 ⅓ cups (160 g) all-purpose flour (plain flour); 1 cup (200 g) white granulated sugar; ⅓ cup (35 g) unsweetened cocoa powder, sifted; ¾ teaspoon baking soda (use a ¼ teaspoon three times, level (each one, too much affects taste and texture)); ¼ teaspoon salt; ⅔ cup (160 ml) buttermilk; ½ cups (120 ml) oil (canola, sunflower, or (vegetable)); 1 large egg, beaten; 1 teaspoon vanilla extract; ⅔ cup (160 ml) hot coffee or hot water ( (around 175°F / 80°C, not boiling)); 1 ½ cups (180 g) powdered sugar (icing sugar), (sifted); ⅓ cup (40 g) unsweetened cocoa powder, sifted; ⅓ cup (85 g) unsalted butter, melted and hot; ¼ cup (60 ml) buttermilk, at room temperature

Jump to Recipe

This Buttermilk Chocolate Sheet Cake is soft, deep in flavor, and covered in a smooth cocoa frosting.
It’s easy to make, but the result is anything but plain. One bowl. Perfect slices that stay soft for days.

Close-up of a square slice of chocolate sheet cake with rippled buttermilk frosting, served on a gold cake lifter.

Updated September 2025: Now tested in a 9×13-inch (quarter sheet, 33 × 23 cm) pan with easier steps, improved photos, and step-by-step video instructions.

Buttermilk Chocolate Sheet Cake

This cake uses oil instead of butter, which gives it a softer texture that holds up for days even after time in the fridge.

Hot water keeps the batter smooth and easy to pour. Freshly brewed or instant coffee works too if you want a deeper chocolate flavor. Totally up to you.

The frosting is simple: melted butter, sifted cocoa, powdered sugar, vanilla, and just enough buttermilk to make it smooth and easy to spread.

How to make Buttermilk Chocolate Sheet Cake

These are the visual steps. For the full recipe with exact measurements, scroll to the recipe card at the bottom of this post

Steps 1 and 2 of making buttermilk chocolate sheet cake: whisking dry ingredients, then adding buttermilk, oil, egg, and vanilla to the bowl.

1. Whisk together flour, sugar, cocoa, baking soda, and salt.
2. Add buttermilk, oil, whisked egg, and vanilla to the dry mix.

Steps 3 and 4 of making buttermilk chocolate sheet cake: pouring in hot coffee or water, then whisking the thin chocolate batter until fully combined

3. Add hot coffee or water and whisk just until mixed, 10 seconds is enough.
4. Whisk well until fully combined, the batter will be thin.

Use hot water (around 175°F/ 80°C ), not boiling, it should be steaming, not bubbling.
If you’re using instant coffee, dissolve the granules in the hot water before adding.

Steps 5 and 6 of making buttermilk chocolate sheet cake: batter poured into a lined sheet pan, then baked until the surface is set and a skewer comes out clean.

5. Pour into a 9 x 13 inch (23 x 33 cm) lined pan.
6. Bake for 18–22 minutes, or until a skewer inserted in the center comes out clean. Remove from the oven and leave the cake to cool completely in the pan.

Don’t try to lift the cake out or turn it onto a rack, it’s soft and will break. Let it cool right in the pan. Don’t worry, it won’t go dense or heavy.
It cools to a soft, light texture.

Make the Frosting

Only make the frosting when the cake is completely cool and you’re ready to use it.
Mixing it too early and leaving it in the bowl will cause it to firm up — and once that happens, it won’t spread smoothly over the cake.

Steps 7 and 8 of making frosting for chocolate sheet cake: hot melted butter poured into sifted cocoa and powdered sugar, then buttermilk added to the clumpy mixture.

7. Sift powdered sugar and unsweetened cocoa into a bowl. Pour in the hot melted butter.
8. Mix to coat the cocoa and powdered sugar, the mixture will look clumpy at this point (that’s fine).

Always sift the cocoa and powdered sugar before mixing the frosting.
Skipping this step leaves clumps that won’t break down, and your frosting won’t turn out smooth.

Steps 9 and 10 of frosting chocolate sheet cake: whisking thick chocolate frosting until smooth, then spreading it over the cooled baked cake.

9. Mix with a whisk or spatula, it’ll look thick at first, but keep going. It transforms into a silky, rich frosting.
10. Spread over the completely cooled cake.

Steps 11 and 12: smooth frosting spread over chocolate sheet cake, then decorated with a wavy lined pattern using a cake decorating comb.

11. You can leave the frosting as is — smooth and simple.
12. Or, if you’re feeling fancy, use a decorating comb (like the one here) to make a lined pattern, straight or wavy.

Just drag it across the frosting in one smooth pull, or give it a gentle wiggle for the wave effect. You can see exactly how it’s done in the video below.

Tip: You can slice and serve while the frosting is soft, or leave it for 20 minutes to set.
For extra neat slices, chill the cake in the fridge for 30 minutes. Because it’s made with oil, the cake stays soft even when cold.

Full chocolate sheet cake in pan, topped with swirled buttermilk cocoa frosting using a comb for a wave pattern.

Storage

Room temperature: Cover and store the cake at room temperature for up to 3 days.
Fridge: For longer storage, refrigerate the cake for up to 5 days. The frosting firms up slightly, but the cake stays soft, thanks to the oil.
Freeze:You can also freeze individual slices, frosted or unfrosted, for up to 1 month. Slice before freezing and place in a single layer in a freezer-safe airtight container. Thaw at room temperature. The texture holds up beautifully.

Side view of a single slice of chocolate sheet cake, showing the soft texture and even crumb beneath the frosted top.

Enjoy ♥

Recipe

Buttermilk Chocolate Sheet Cake

PREP TIME 20

COOK TIME 22

cooling 1

TOTAL TIME 1 42

12 slices

This Buttermilk Chocolate Sheet Cake is soft, deep in flavor, and covered in a smooth cocoa frosting.

Video

Ingredients

For the cake

1 ⅓ cups (160 g) all-purpose flour (plain flour)
1 cup (200 g) white granulated sugar
⅓ cup (35 g) unsweetened cocoa powder, sifted
¾ teaspoon baking soda (use a ¼ teaspoon three times, level each one, too much affects taste and texture)
¼ teaspoon salt
⅔ cup (160 ml) buttermilk
½ cups (120 ml) oil (canola, sunflower, or vegetable)
1 large egg, beaten
1 teaspoon vanilla extract
⅔ cup (160 ml) hot coffee or hot water (around 175°F / 80°C, not boiling)

For the frosting

1 ½ cups (180 g) powdered sugar (icing sugar), sifted
⅓ cup (40 g) unsweetened cocoa powder, sifted
⅓ cup (85 g) unsalted butter, melted and hot
¼ cup (60 ml) buttermilk, at room temperature

Instructions

Lightly grease a 9×13-inch metal baking pan (quarter sheet) 33 × 23 cm) and line just the bottom with parchment.
Preheat the oven to 350°F (175°C) or 160°C fan.

Make the cake

In a large bowl, whisk together the flour, sugar, sifted cocoa powder, baking soda, and salt.
Add the buttermilk, oil, beaten egg, and vanilla extract. Whisk just until combined.
Pour in the hot coffee or hot water
Tip: If using instant coffee, dissolve the granules in the hot water before adding
Whisk until the batter is fully combined. It will be liquidy.
Bake: Pour into the prepared pan. Bake for 18–22 minutes, or until a skewer inserted in the center comes out clean.
Cool: Leave the cake to cool completely in the pan.
Tip: Don’t lift or turn out the cake, it’s soft and can break. It cools to a light texture right in the pan.

Make the frosting

Tip: Only make the frosting once the cake is completely cool and you're ready to use it. It sets quickly and won’t spread well if made too early.
In a bowl, sift the powdered sugar and cocoa powder.
Tip: Skipping this step can leave clumps that won’t smooth out.
Pour in the hot melted butter, and buttermilk.
Whisk until smooth and glossy. The frosting may look thick at first, but keep mixing, it turns smooth and spreadable quickly.
Spread evenly over the cooled cake with a spatula.
Optional: Use a decorating comb to make a wave or line pattern. See video above for technique.

Recipe Notes

Baking soda: Also known as bicarbonate of soda (UK). Use a ¼ teaspoon three times and level each one carefully. Avoid heaping or guessing — too much baking soda can affect the taste and texture.

Nutrition

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Comments

5 from 3 votes

Buttermilk Chocolate Sheet Cake

Buttermilk Chocolate Sheet Cake

How to make Buttermilk Chocolate Sheet Cake

Make the Frosting

Storage

Recipe