Roughly chop the chocolate. Peel and dice the bananas and mix with the lemon juice. Mix the soft butter, powdered sugar and salt with the hand mixer for 8 minutes until very creamy. Stir in eggs one at a time. Mix the flour and baking powder, mix with the nuts and the chopped chocolate. Then mix with the egg mixture. Finally fold in the banana cubes. Put everything in muffin molds and bake in the preheated oven for about 25-30 minutes.