Prehear oven to 340F (170C) and line an 8x8 brownie pan with parchment paper.
Measure the cocoa powder and milk powder in a large bowl and pour over the warm melted butter and vanilla, mix to blend.
Mix the brown sugar and granulated sugar followed by the eggs.
Add the flour and the chocolate chips, and pour the batter into the prepared pan.
Bake for 20-22 minutes or until a toothpick comes out of the center of the cake with moist crumbs.
Remove from the oven and immediately spread over the mini marshmallow. Let the brownie cool and melt the marshmallow from the heat residue.
Slice when cooled into 16 or 12 pieces and serve.