Preheat oven to 350F and line a 9x13 pan with parchment, leaving an overhang on two sides.

Melt butter in a large bowl (microwave in 30-second bursts), then whisk in sugar until combined.

Whisk in eggs one at a time, then vanilla, until the batter looks glossy.

Fold in cocoa powder, flour, salt and baking powder just until no dry streaks remain, then stir in chocolate chips.

Spread batter evenly in the pan.

Bake 26 to 30 minutes, until a toothpick inserted 2 inches from the edge comes out with a few moist crumbs (center will firm as it cools).

Cool brownies completely in the pan, about 30 minutes, then lift out using the parchment overhang.

Beat butter, powdered sugar, vanilla, milk and green food coloring into a smooth buttercream, thick enough to hold a swirl.

Spread green buttercream evenly over the cooled brownies.

Pipe white buttercream yard lines across the field using a small round tip.

Cut brownies into 12 bars and press a mini pretzel stick upright at the corners for goalposts.

Place a chocolate football candy with white icing laces on 2 or 3 bars to finish.