Chocolate sauce:
1/2 cup cocoa powder
125g butter
3/4 cup milk
1/2 cup sugar
Cake batter:
4 eggs
3/4 cup cake flour
1/4 cup sugar
2 tbsp oil
1 1/2 tsp baking powder
Make chocolate sauce: Melt butter, milk and sugar in a sauce pan, add cocoa powder and stir continously. Stir until all the ingredients have dissolved and the mixture is liquid without any lumps. Leave to cool.
Make cake batter: Separate egg whites and yolks. Beat egg whites until soft peaks, then add sugar and combine beating until stiff peaks. Beat yolks until pale yellow, and then add the oil and continue beating until well-mixed. Sift together flour and baking powder. Fold in the egg yolks and flour into the egg white mixture. Fold until the batter is paste-like and runny, but be careful not to lose the air in the egg whites. Finally, add the chocolate sauce to the cake batter and continue folding until well-mixed. Pour the mixture into a baking pan (You can use a spoon to swirl the chocolate sauce to make it look nice). Bake the cake in an oven preheated at 350F for about 45 minutes.