Preheat oven to 375 degrees. Line 18-20 muffin cups with paper liners. Spray liners with non-stick cooking spray.

In a large bowl, combine flour, sugar, chocolate chips, cocoa powder, baking powder and salt. In a small bowl, whisk together chocolate milk, yogurt, eggs, vegetable oil and vanilla until smooth. Add wet ingredients to dry ingredients and mix until batter is just blended. Stir in mashed bananas.

Fill prepared muffin cups 3/4 full. Bake for about 20 minutes, or until a toothpick inserted into the center comes out clean. Cool in the pans for 10 minutes before removing to a wire rack to cool completely.