Pour the melted butter and the sugar into the mixing bowl. Stir together until well blended.

Add the eggs and mix well. In a separate bowl mix the flour, the cacao, the baking powder and the baking soda.

Sift the dry ingredients and add them to the mixing bowl, one spoonful at a time.

Add milk (room temperature). Break the chocolate into small pieces (approximately 0.5mm) and add them all to the mixture. Stir everything together with a spatula, then put the mixture inside of a nozzle-less pastry bag.

Fill each cup of a muffin pan with the batter. Bake for 30 minutes in the oven at a moderate temperature (180 °C). Once taken out of the oven, fill the muffins with the blueberry spreading cream using a baking syringe.