In a medium bowl, melt 1/2 cup unsalted butter until just melted. Let it cool slightly.

Whisk in 1/2 cup granulated sugar and 1/4 cup brown sugar until smooth.

Add 1 large egg and 1 teaspoon vanilla extract. Whisk until fully combined.

Sift in 1/3 cup unsweetened cocoa powder, 1/2 cup all-purpose flour, and 1/4 teaspoon salt. Stir just until combined.

Fold in 1/2 cup mini chocolate chips.

Preheat oven to 350°F (175°C) and grease a 24-cup 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.