Preheat the oven to 350°F and grease a 9x9-inch baking pan, lining it with parchment so the ends hang over two sides.

Whisk the melted butter and sugar together in a large bowl until the mixture looks glossy.

Whisk in the eggs one at a time, then the vanilla, until the batter is smooth.

Stir in the cocoa powder, flour, salt and baking powder just until no dry streaks remain; a few extra stirs make the brownies tough.

Pour the batter into the pan and smooth the top with a spatula.

Bake for 25 to 28 minutes, until the edges have pulled slightly away from the pan and a toothpick inserted in the center comes out with a few moist crumbs, not wet batter.

Cool the brownies in the pan for 15 minutes, then lift them out by the parchment and cool completely on a rack, about 1 hour, before cutting.

Press a 3-inch heart-shaped cutter firmly into the cooled brownie slab to stamp out as many hearts as you can.

Spoon the frosting into a piping bag fitted with a small round tip, or a zip bag with the corner snipped off.

Pipe one line down the center of each heart and short lines crossing it, just like laces on a football, then let the frosting firm up for 10 minutes before stacking.