Ingredients

• 2 cups (440 g) sugar
• 1 cup + 2 tablespoons (140 g) butter, softened
• 2 cups (220 g) all-purpose flour
• 7 tablespoons unsweetened cocoa powder
• 1 teaspoon baking soda
• 1 teaspoon vanilla extract
•  1 cup boiling water
• For the frosting:
• 1 cup water
• 4 egg whites
• 1 pinch salt
• 1 teaspoon vanilla extract

Method

1. Preheat oven to 350 deg F (Gas mark 4 or 180 deg C) and grease two 9 in (23 cm) cake tins.
2. Put cocoa in small bowl; add boiling water and mix until smooth. Let cool.
3. Using an electric mixer, cream the butter and sugar on medium speed for approximately 5 minutes. Add egg yolks and mix well.
4. Sift in flour, baking powder and baking soda, add cocoa mixture, sour cream, and vanilla extract and mix well.
5. Whisk egg whites until stiff and fold gently into the cake mixture. Divide into two equal portions and transfer to the cake tins.
6. Bake at 350 degrees until a wooden pick inserted in center comes out clean, approximately 40-50 minutes. Let the cakes cool.
7. Slice each cake through the middle to make altogether four layers; if necessary remove the top of the cake by using a knife.
8. For the frosting, combine sugar and water in a saucepan. Cook until candy thermometer reaches 242 degreesF (115 degrees C) or until a teaspoon of mixture dropped into cold water forms a ball when removed from water. Add cream of tartar and a pinch of salt to the egg white, and beat them until stiff. Add hot syrup in a slow stream, beating constantly. Stir in vanilla. Use approximately one fifth of the frosting between each layer of the cake, coat the cake with the remaining two fifths of the frosting.
9. Cool the cake in the fridge. Place the cake at room temperature 1-2 hours before it is served.