Preheat the oven to 400°F. Prepare your muffin pan by placing 16 cupcake liners or spray with nonstick baking spray.

Cream the butter and light brown sugar together in a mixing bowl until light and fluffy.

Add the egg, vanilla extract, mashed banana, and sour cream to the bowl. Beat until well combined.

Mix the dry ingredients by stirring flour, baking powder, baking soda, salt, cinnamon, and graham cracker crumbs in a separate bowl.

Combine the dry ingredients and milk in batches until just combined.

Fold in the miniature chocolate chips and marshmallow bits gently by hand.

Fill each muffin liner with the batter, leaving room at the top. Sprinkle remaining chocolate chips and marshmallow bits on top.

Bake for 16 minutes or until golden brown and a toothpick comes out clean.

Cool in the pan for about 3-4 minutes, then transfer to a wire rack.

Store the muffins in a sealed container to keep them fresh.