Preheat oven to 350°F (175°C). Line a 9×13-inch baking pan with parchment paper, leaving a little overhang for easy removal.

In a large bowl, whisk melted butter and sugar until smooth. Add eggs one at a time, mixing well after each. Stir in vanilla extract.

In another bowl, sift together flour, cocoa powder, salt, and baking powder. Gently fold into the wet mixture until just combined.

Stir in dark chocolate chips for extra richness.

Divide batter in half. Tint one portion green and the other purple. Drop alternating scoops into the pan, then swirl with a toothpick for a marbled look.

Bake 25–30 minutes, until a toothpick inserted in the center comes out with a few moist crumbs.

Let brownies cool completely. Cut into squares and top with Halloween sprinkles if desired.