Preheat oven to 325°F (160°C) and line an 8x8-inch baking pan. Combine melted butter and graham cracker crumbs in a medium bowl and press into pan.

In a microwave-safe bowl, combine remaining butter, sugar, cocoa powder, and salt. Microwave until melted and smooth, about 1-2 minutes. Let cool slightly.

Stir in vanilla and add eggs one at a time, mixing until thick and glossy. Gradually incorporate flour and fold in chocolate chips.

Pour half the brownie batter into the prepared pan. Add dollops of marshmallow fluff and swirl. Top with remaining batter and more fluff.

Bake for 25 minutes, increase temperature to 375°F (190°C), add mini marshmallows, and bake for an additional 3-5 minutes until golden brown.

Let the brownies cool completely before slicing into squares.