Grease and line a 13 by 9-inch (33 x 23 cm) baking pan. See note 7
Preheat the oven: Preheat the oven to 350°F/180°C (160°C fan)
Combine eggs and yolks, buttermilk, and vanilla in a bowl or measuring jug and whisk or beat until smooth. Set aside
In a separate large bowl, whisk together the cake flour, sugar, baking powder, baking soda, and salt.
Add butter and oil and mix on low speed until combined, about 30 seconds. The mixture will have a crumbly texture.
With mixer running, slowly add egg mixture.
When mixture is fully incorporated, stop mixing and scrape down bowl with a rubber spatula.
Beat on medium-high speed until the batter is pale, smooth, and thick, about 3 minutes.
Transfer the batter to the greased and lined pan and smooth the top. Tap the pan firmly on counter to release any large air bubbles.
Bake until toothpick inserted in center comes out clean 25 to 30 minutes.
Turn out the cake onto a wire rack and remove the baking paper.
Allow to cool completely before frosting.
Make the chocolate frosting
Beat the butter on medium speed with a hand mixer until it is smooth, about 1 minute.
Add the powdered sugar and cocoa powder to the bowl and beat until combined.
Pour in the milk and vanilla extract and continue beating until well combined, scraping down the sides as needed, about 2 minutes.
Frost the cake
Turn the cooled cake back into the baking pan and spread the frosting over the cake.
Leftovers can be stored covered at room temperature for up to 3 days or in the fridge for up to 5 days.
Make ahead tip
The cake and frosting can be made up to three days ahead, refrigerated, and the cake frosted the day you wish to serve it. For storing, wrap the completely cooled cake in cling wrap and transfer the frosting to an airtight container.
When you're ready to decorate the cake, remove the frosting from the fridge and warm at room temperature until soft enough to spread easily.
1The extra yolks add richness, decadence, and lightness to the cake.
The buttermilk in this recipe serves to add moisture and keeps the sponge soft, light, tender, and fluffy. If you don’t have buttermilk, you can make some by adding 1 tablespoon (15ml) white vinegar or lemon juice to 1 cup (250ml) milk Leave it aside for 2 minutes which will allow it to curdle. Use in place of the buttermilk.
It's important that you use bleached cake flour in this recipe creating the finest texture and the best rise. Unbleached cake flour or all-purpose results in a dense bake and a sinking cake.
This recipe has a combination of butter and oil to give off that nice buttery taste while the oil keeps it soft and moist for a longer time. Cakes using pure butter is denser and drier compared to adding oil into the batter. I use canola oil for this vanilla cake but you may substitute it with vegetable or sunflower oil.
It's important that you sift the powdered sugar and unsweetened cocoa powder for a smooth silky frosting.
Milk is used for thinning out the frosting and contributing to a smooth consistency making it easier to pipe or spread.
Use a metal baking pan: I prefer to use a metal baking pan as opposed to a glass pan. Glass pans absorb heat so they bake more quickly than a metal pan. It's easier to over-bake a cake in a glass pan, because it takes longer for the center to cook. By the time the center finishes, the cake is over-baked, and the outer edges dark brown or simply burnt.