Puree, mix or blend bananas until smooth. Add in butter and yogurt and mix until well combined. Add in eggs and vanilla, mixing until well combined. Stir in sugar until well combined. In a separate bowl, mix white whole wheat flour, dark cocoa powder, baking soda and salt. Combine wet and dry ingredients, add in milk and stir until well combined. Scoop into a muffin pan that has been lightly sprayed with cooking spray. Bake for 15-20 minutes or until tops spring back when touched. Remove from oven and allow to cool. Store loosely covered or in refrigerator.