Ripe (required!) banana mash with a fork until puree state.
In a bowl with eggs and sugar, add salt, vanilla, baking powder and mix well. Then add the butter and sour cream and bring to a homogeneous state. Now we begin to gradually add the sifted flour, it may require a little more or a little less. Should get the dough consistency of thick sour cream.
Divide the dough in half (by eye). in One part add cocoa powder and mix well.
Spread the dough in cupcake molds, I usually use small ones. If they are silicone, then you do not need to lubricate anything. In each mold, put a tablespoon of light dough, and a spoon of dark. Fill approximately 2/3 of the volume.
Now take a match or toothpick and mix the dough in molds.
Put with preheated to 180 degrees oven and bake until tender. It takes 25-30 minutes. Check readiness with a toothpick. Bon appetit!