Preheat the oven to 350°F. Grease and line an 8½ x 4½-inch loaf pan with parchment paper.
In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth.
Add the eggs and vanilla extract, whisking until fully combined. Stir in the mashed bananas.
In a separate bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.
Gradually add the dry ingredients to the wet ingredients, stirring just until combined. Be careful not to overmix.
Fold in the chocolate chips and chopped milk chocolate.
Pour the batter into the prepared loaf pan and smooth the top. Bake for 45 minutes.
Carefully remove the loaf from the oven and sprinkle the mini marshmallows evenly over the top. Return the loaf to the oven and continue baking for 10–15 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
If you'd like extra toasted marshmallows, broil the loaf for 30–60 seconds, watching it very closely so the marshmallows don't burn.
While the loaf is still warm, sprinkle the remaining chopped chocolate over the marshmallows. Let it sit for 2–3 minutes until softened, then gently spread it slightly with the back of a spoon if desired.
