In a medium bowl, melt the unsalted butter until just melted. Let it cool slightly.

Whisk in the granulated sugar and brown sugar until smooth.

Add the egg and vanilla extract, and whisk until fully combined.

Sift in the cocoa powder, flour, and salt, then stir just until combined.

Fold in the mini chocolate chips.

Preheat oven to 350°F (175°C) and grease a mini muffin pan. Divide the batter evenly (about 1 tablespoon per cup).

Bake for 10-12 minutes or until tops are set.

Remove pan from oven and gently press one mini marshmallow onto each brownie.

Broil for 1-2 minutes until marshmallows are toasted. Watch carefully!

Let cool in pan for 5 minutes, then transfer to a wire rack.