Preheat the oven to 370 F (190C).
Mash the banana with a fork in a large bowl. Then add the buckwheat flour, eggs, cinnamon, baking powder, milk, apple, and mix well.
Grease the muffin tin and evenly fill 12 moulds with batter, sprinkle the dark chocolate on top. Bake in the oven for about 25 minutes. Leave to cool before serving.