Preheat the oven to 350 F and line an 8x8 inch baking pan with parchment, leaving an overhang on two sides.

Whisk the melted butter and granulated sugar together in a large bowl until the mixture looks glossy, about 1 minute.

Beat in the eggs one at a time, then the vanilla, until the batter is smooth and slightly pale.

Fold in the cocoa powder, flour and salt just until no dry streaks remain, the batter should be thick and shiny.

Spread the batter into the prepared pan and bake for 26 to 28 minutes, until a toothpick comes out with a few moist crumbs.

Cool the brownies completely in the pan, about 45 minutes, before lifting them out by the parchment.

Beat the powdered sugar, softened butter and milk together until smooth, then add green food coloring a drop at a time until bright.

Cut the brownies into 16 squares, pipe or spread a swirl of green frosting on each, and press one red heart sprinkle into the center.