Separate the whites from the yolks. Put the whites in the refrigerator. In a deep bowl, mix the yolks, vanilla sugar and 100 grams of granulated sugar. Whip the mixture until it turns white and increases in volume.
Remove the whites from the refrigerator and beat with the remaining granulated sugar until coarse foam forms.
Mix the whipped whites with the yolks. Sift the flour into the egg mixture, stirring the mixture gently.
Preheat the oven to 180 ℃. Grease the baking dish with oil. Pour the dough into it. Bake for 30-40 minutes. After baking, let the biscuit cool.
While the sponge cake is baking, prepare the cream. Pour the cooled cream into a bowl and beat with a mixer at low speed.
Add powdered sugar and vanilla in small portions and beat until smooth. The mass should become dense and increase in size.
Assemble the finished cake. Cut the sponge cake lengthwise into 2 parts.
Place one cake layer on a large plate and spread most of the buttercream on top.
Cover with the second cake layer, spread the remaining cream on the top and sides of the cake.