Grease, line and lightly flour an 8 inch / 20cm cake pan. Dust out excess flour and keep aside.
EGG YOLK MIXTURE
Heat the butter a saucepan, over low heat.
Add milk and sugar and a pinch of salt and stir until the sugar dissolves. Turn off the heat, sift in the cake flour, add the egg yolks and mix by hand, until just combined. do not over mix
EGG WHITE MIXTURE
In a separate bowl, Whisk egg whites until foamy, squeeze lime or lemon juice and add sugar a little bit at a time, beating until stiff peaks form. See note 1
COMBINE EGG YOLK MIXTURE WITH EGG WHITE MIXTURE
Add about 2 Tablespoons of the egg white mixture into the egg yolk mixture.
Mix until just combined and then pour this combined batter into the remaining egg white mixture It is important that you gently fold this batter until combined. Do not stir or whisk vigorously, just fold the mixture.
Pour the batter into the greased and lined cake pan. Tap the cake pan on the countertop a few times and get the air bubbles out of your cake batter.
PREPARE THE WATER BATH
Prepare the water bath by pouring cold water into a large pan. I use a large roasting pan for this purpose. Place the cake pan containing the batter into the large roasting pan The water in large baking pan should reach 1 inch / 2 ½ cm up the sides of cake pan.
Bake in preheated oven 302°F /150°C for 20 to 25 minutes or until the cake is firm and set in the center. Baking time is just a guide,as oven temperatures and cake tin sizes vary.
If you find that the cake is browning too quickly before it has been baked, then just cover loosely with foil. Do not let the foil touch the batter.
Once baked, remove from the oven and water bath. Leave the cake in the pan to cool for about 10 minutes before gently turning out. Cool completely and dust with powdered sugar or sandwich and cover with a frosting of your choice.
Frequently asked questions about this recipe
Can I double this recipe? See note 2
Do I whisk or fold in the egg whites? See note 3
What is the water bath method and how does it help? See note 4
Does the cake deflate when removed from the oven? See note 5
Beating the egg whites to a stiff peak is one of the important factors that create that soft fluffy perfection when baked. When you lift the whisk, the tip of egg whites go straight up and then fold over. It’s stiff enough to stand up, yet the tip is soft that it folds over. Avoid overbeating at this stage. Overbeating the egg whites will cause it to have a curdled appearance and is not usable.
If you are going to make more than one layer of this cake, I suggest that you make the batter in separate batches. Avoid doubling or tripling the recipe.
Be gentle when folding the egg whites into the batter Use a spatula to fold in the egg whites into the batter. It is important to use light movements so that you do not deflate the beaten egg whites. If it is deflated, then the cake will be dry and dense.
This sponge cake is baked using the water bath method. This method consists of placing the cake pan with the batter in a large, shallow pan of warm water. The water surrounds and protects delicate sponge during baking by maintaining an even, low-moisture heat. It also keeps the surface of the cake level and prevents the cake from cracking. Pour the batter into prepared pan, and put cake pan into a large baking pan ( I use a large roasting pan for this purpose.) The water in the large baking pan should reach 1 inch / 2 ½ cm up the sides of the cake pan.
The cake is going to deflate when you take it out of the oven. I am using an 8 inch / 20cm round pan for the cake. After baking, as it cools it will deflate and be approximately 2inches / 5cm in height. If you do want more height, then I suggest that you make a separate batch of batter, bake it in 2 pans and sandwich it with a frosting of your choice.