Preheat oven to 200°C (400°F)
Melt butter and allow to cool.
Sift together flour, cocoa, baking powder and baking soda. Add the sugar and stir.
In another bowl, finely grate the banana and mashed it well to obtain a puree.
Add 2 eggs and mix well.
Incorporate the butter while continuing to whisk, then add the yogurt. Mix well.
Add the flour-cocoa mixture (dry ingredients) and mix well. The dough is thick.
Mix with a spatula to combine all ingredients. Don't try to lighten the dough by whisking it.
Add the chocolate chips.
Fill your muffin cups with a small spoon to about 3/4 full.
Put a few chocolate chips on top of each muffin. I always place the cups in a muffin tin.
Bake for 15 to 20 minutes, depending on the size of the muffins and the oven. A skewer inserted into the center of the muffin should come out dry. Be careful not to overcook them, or they will dry out.