Prepare the cupcake batter. Take flour, baking powder and soda. Mix well and sift through a fine sieve into a deep bowl. Melt the butter. Make sure it is not too hot. Hot butter causes the egg white to curdle. Mix the melted butter, eggs and sugar in a separate bowl. Mash the banana until smooth and add it to the egg mixture.
Add sour cream to the resulting mixture with banana puree and mix thoroughly. Combine the mass with the dry flour mixture, mix until smooth. Grind the nuts and add them to the dough. Leave a small amount to decorate the finished dessert.
Bake the cupcakes in the oven. To do this, preheat it to 180 degrees. Place paper cupcake liners in a special baking pan. Pour the batter 2/3 full into each pan. Place the pan in the oven and bake for 20-25 minutes. Check for readiness with a match. If it remains dry, the cupcakes can be taken out of the oven.
Make the cupcake frosting. Whip the mascarpone cheese with the powdered sugar and use a pastry bag to make beautiful swirls on the cupcakes.