Melt butter in a small saucepan over moderately low heat, then remove from heat.
Whisk in milk, then whisk in whole egg, yolk, and vanilla until combined well.
Whisk together flour, sugar, baking powder, cocoa powder, and salt in a bowl, then add milk mixture and stir until just combined. Notice: NEVER over mix the ingredients, for muffins, the best way is to mix by hand with a fork :).
Cut banans into small cubes, and then fold them in and mix slightly.
Divide batter among the muffin cups, spreading evenly. I tend to fill the cups a bit full so that the muffins will get over the cup top after baking - just like what we get from the shops!
Bake about 18 to 20 minutes, or until a toothpick inserted in the center of a muffin comes out clean.
Open the oven a bit to let the muffins settle before move out. Then cool in pans on a rack 15 minutes.