Peel the bananas and mash them with a fork until smooth.

Cream the softened butter with a whisk, gradually adding sugar in three parts until light and fluffy.

Beat the egg and slowly incorporate it into the butter mixture. You can add yolk first and then whites for better consistency.

Sift the flour, cocoa powder, and baking powder into the mixture. Fold gently using a spatula in a cutting motion.

Add the milk and mix gently using the same folding technique.

Add the mashed bananas and mix until well combined.

Prepare the muffin tins with parchment paper and fill them 70% full with the batter. Top each muffin with a slice of banana.

Bake in a preheated oven at 170°C for 20-25 minutes. Test with a toothpick; if it comes out clean, the muffins are ready.