Banana-chocolate muffins with a shade of lemon

Here's a recipe for muffins of sophisticated taste characterised by the richness of dark chocolate (although I don't use the darkest one possible), the sweetness of banana and the freshness of lemon.

Ingredients (x12 muffins):
150g butter or margarine
2 dl sour cream
zest of one lemon
2 eggs
1 ½ dl sugar
1 teaspoon vanilla sugar
3 dl wheat flour
1 dl cocoa powder
2 teaspoons baking powder
a pinch of salt
3 bananas
200 g dark chocolate

Preparation:
Heat up the oven to 175 c. Melt the butter and add into it the sour cream and the lemon zest. Mix together the eggs, sugar and vanilla sugar so that the final result is light and airy. Mix the sourcream-butter mix into it. Mix in another bowl the wheat flour, cacao powder, baking powder and salt and add this flour mix into the dough. Slice the bananas and cut the chocolate into pieces and add them into the dough. You can however set aside some pieces of clocolate in order to decorate the surface of the muffins with them.

Fill cake cups with the dough. Bake in the middle level of the oven for 15-20 minutes.