Grind the yolks with sugar. To do this, add the yolks, sugar, salt and vanilla to a deep bowl. Beat the mixture with a mixer until it becomes smooth and fluffy.
In a separate container, beat the whites. They should increase in volume by 2.5-3 times. Add them in portions to the yolk mass, alternating with sifted flour and stirring gently.
Pour the dough into the form and smooth out its surface. Place the form in the oven and bake the sponge cake until golden brown, about 35 minutes. You can check the readiness of the sponge cake with a wooden toothpick. Pierce the cake with a toothpick, there should be no dough left on it.
Leave the sponge cake in the pan for 7-10 minutes. Then carefully, trimming the sides with a knife, remove it from the springform pan. Cool on a wire rack.
Prepare the cream. Boil the milk and cool it to room temperature. Mix it with vanilla. Beat the butter with a mixer and, without stopping, add the powdered sugar. Pour in the cooled milk, gradually increasing the speed of beating.
Start assembling the cake. Cut the cake into two parts lengthwise, it is most convenient to do this with a special string or a long serrated knife. Coat the cooled cakes with cream, smooth out and assemble the cake. Coat the sides of the cake with the remaining cream. Sprinkle the top of the cake with cocoa powder and put it in the refrigerator for a few hours.