Combine eggs and sugar and beat until light in color (2-3 minutes).
Add sour cream, flour, baking powder, and sweetened condensed milk to the egg mixture and mix well until the mixture is smooth.
Take ⅔ of the mixture and spread evenly on one baking sheet (my sheet was 10.5 x 15 in. in size).
Take the leftover batter and add the cocoa powder. Mix well and pour out on a second baking sheet.
Let batter stand for 15 minutes. In the meantime, heat the oven to 350° F (180 °C). Bake both sheets for 15-20 minutes (see note below about ovens). Let cool.
White frosting
Mix sugar and sour cream together. Slowly add the sweetened condensed milk while mixing.
Spread half of the frosting on the white cake layer.
Break the black cake layer into small pieces (about 1 inch in size) and drop them allover the white cake layer.
Drop spoonfuls of the white frosting randomly all over the black cake pieces of the cake.
Black frosting
Heat the butter in a small saucepan. Add sugar, cocoa, sour cream, and the rest of the sweetened condensed milk. Let everything cook for a few seconds.
Drop spoonfuls of the black frosting on the top of the cake.
Let everything cool well. Serve cold.
Notes
A note about ovens. In Germany, most of the ovens have what's called an Umluft function, or a fan-assisted function. Generally, when using this function the temperature needs to be lower (in this case it should be 320° F (160° C), because the fan makes the oven hotter). The fan also allows for multiple things to be baked at the same time. You might need to check your oven if this is a possibility. If you don't have such a function on your oven, bake the sheets separately at 350° F for 15-20 minutes. You might also need to increase the time as normal ovens take a bit longer to bake. You can always check the done-ness of the cake by inserting a toothpick to see if it comes out clean. To read more about this topic, check out this link Is a Fan Assisted Oven Better.