Line a baking sheet with parchment paper or aluminum foil. Place the unpeeled bananas on the baking sheet and bake for 15 minutes (the peels will turn dark and there may be some liquid that leaks out). Remove the bananas from the oven and let cool. Reduce the temperature to 350 degrees.
In a large bowl, mix together the flour, sugar, baking powder, and salt.
Once cool, peel the roasted bananas and mash. In a medium-sized bowl, mix together 1 cup mashed bananas, egg, butter, milk, and vanilla and stir until combined. Add the banana mixture to the dry ingredients and mix until just combined (do not overmix). Stir in the chocolate chips.
Line a 12-cup muffin tin with paper or foil liners. Divide the batter into the muffin cups, filling up approximately ¾ full. Bake in the oven until a cake tester comes out clean (with possible melted chocolate), about 30 minutes. Remove from the oven, transfer the muffins to a wire rack, and let cool.