Mix flour with baking powder and sift. Beat eggs with sugar and vanillin for 7 minutes. During this time, the mass should whiten and expand significantly. Then gradually pour in the flour and baking powder and stir the dough until smooth with a spatula. You need to do this carefully, so that the cake turns out to be fluffy. Pour the resulting mass into the pre-oiled and floured loose-base cake tin (the optimal diameter is 22-24 cm). Bake at 180 °C for approximately 40 minutes. You can also bake the cake in a multi-cooker in the "baking" mode: in this case the process duration will depend on its parameters. To check if the sponge is done, stick it with a toothpick. If the toothpick is dry after taking out, then the cake is baked. Leave the sponge for a few hours. It should be done to prevent the cake from later crumbling when cutting. Make the custard. Beat up sugar, flour, milk and egg until smooth. Put the pan of this mixture on a low heat and continuously stir the contents until it thickens (5-7 minutes). Then add butter (previously softened at room temperature) to the warm mixture. Beat up again. The resulting cream should be dense and thick. Cover the mass with a cling film, so that no crusts form on the surface, and allow to cool completely. Do not stir the cream during this time. Combine the cake and the cream. Cut the sponge into two parts and evenly coat one of them with cream. Cover with the second part. You can spread some residues of the cream atop the cake: so the glaze will lie more evenly. Make a chocolate glaze. Simmer cocoa, sour cream and sugar without boiling and stirring constantly. Add butter softened at room temperature and stir until smooth. By the way, you can make the glaze in a simpler way: melt pieces of dark chocolate with butter on a water bath. Pour the slightly cooled glaze over the cake and evenly distribute it with a spatula or brush. Then refrigerate the cake for a few hours, or better for a night.