Separate the whites from the yolks. Beat the whites with a mixer (for 2 min.). Add sugar, condensed milk, sour cream and beat for 3 more minutes. Separately beat the yolks to white. Sift out flour together with a baking powder and cocoa into a large bowl. Carefully mix the whites with the beaten yolks and flour. Stir with light movements from the bottom up. Preheat the oven to 200 °C. Oil the baking tin with a diameter of 25 cm. Pour in the dough, place it in the oven and bake for 30 minutes. (If you use a one-piece tin, then oil the bottom and the walls and slightly sprinkle with semolina.) So it will be easier to take out the finished sponge-cake). Prepare the cream while the cake is being baked. Whip up a softened butter with cocoa with a mixer until smooth (for 1 minute). Pour in the condensed milk and whip for 2 more minutes. *If the cream is not sweet enough, you can add sugar powder or sugar syrup to taste. Take the finished sponge-cake out of the oven. Put the baking tin on a wet towel and cover with a dry one. Allow to cool. Take the cooled sponge-cake out of the tin and cut into 3 cakes identical in thickness. Saturate the cakes with vodka. Coat two cakes with cream. Put them on top of each other and cover with a third cake. Do not cream the upper cake. Chop the chocolate into small pieces, put it in a fire-resistant bowl and place on a pot of boiling water. When the chocolate melts, cover the surface and sides of the cake with it. Refrigerate the cake for at least 3 hours. Also refrigerate the bowl with the remains of melted chocolate. When the chocolate freezes, scrape it off with a small knife. Decorate the cake with chocolate chips.