Prepare muffin batter:

For the muffins: In a large bowl, use an electric mixer to cream butter and sugar together until light and fluffy, approximately 3 minutes. Add eggs one at a time, beating thoroughly after each addition. In another bowl, combine flour, cocoa powder, baking powder, and salt with a whisk. Incorporate the dry mixture into the butter mixture, mixing just until combined. Add milk and stir until smooth. Gently fold in dark chocolate chips.

Fill muffin pan:

Heat oven to 350°F (175°C) conventional or 320°F (160°C) fan. Rinse a silicone muffin pan with cold water and allow to drain without drying; alternatively, butter a 12-cup metal muffin pan. Distribute batter evenly across the 12 cups.

Bake and cool muffins:

Bake for approximately 25 minutes, or until a toothpick inserted into the center emerges clean. Allow muffins to cool fully in the pan on a wire rack. Once cooled, take muffins out of the pan. Slice off the top third (the dome) from each muffin. Crumble all domes into fine crumbs by hand and set aside in a bowl.

Whip cream and slice banana:

For the topping: In a chilled bowl, whip heavy cream with vanilla sugar until stiff peaks form. Keep refrigerated until use. Peel the banana and cut into thin rounds. Arrange 2-3 slices atop each muffin base.

Assemble cream topping:

Using a small offset spatula, spread whipped cream over the banana slices and across the muffin top, smoothing the sides to keep banana slices in place. Form the cream into a dome shape.

Coat with crumbs and chill:

Gently press the reserved cake crumbs onto the cream dome, ensuring complete coverage. Repeat for all muffins. Chill for at least 60 minutes before serving to allow setting.