Place an oven rack in the center of the oven. Preheat your oven to 350°F and line a 12-cup muffin pan with paper liners.
In a small microwave-safe bowl, melt the butter and chocolate chips in 30-second intervals, stirring between each, until fully melted and smooth. Set aside to cool slightly.
4 TBS (56 g) unsalted butter, 2 TBS dark chocolate chips
Whisk together the flour, cocoa powder, baking powder, baking soda, and salt in a medium bowl.
1 ½ cups (180 g) whole wheat flour, 2 TBS (16 g) cocoa powder, 1 tsp baking powder, ½ tsp baking soda, ½ tsp kosher salt
In a large bowl, combine the mashed bananas, yogurt, milk, brown sugar, eggs, and vanilla extract until well combined.
1 cup (227 g) mashed ripe banana, ½ cup (110 g) plain yogurt, ½ cup (120 g) whole milk, ¼ cup (50 g) brown sugar, 2 medium eggs, 1 tsp vanilla
Add the cooled melted chocolate and butter mixture to the wet ingredients and whisk until fully incorporated.
Gently fold in the dry ingredients into the wet ingredient mixture, stirring just until combined. Be careful not to overmix.
Fold in the chocolate chips, save some for sprinkling on top.
½ cup dark chocolate chips
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.Sprinkle the remaining chocolate chips on top of the muffins if you want.
Bake for 20-22 minutes, until domed and set.
Allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.