Start by dividing your American buttercream frosting into seven equal portions. Create two portions of green frosting by mixing green and yellow food gel until completely combined. For black frosting, mix one portion with cocoa powder, milk, and black food gel until smooth. Color the remaining portions accordingly: light yellow, ½ portion yellow, ½ portion red, and one portion purple. Leave one portion white for later use.

Preheat your fan-forced oven to 175°C (340°F). Take a 32 cm x 24 cm baking tin, spray it with non-stick cooking spray, and line the bottom with parchment paper.

Combine the melted unsalted butter and dark chocolate in a microwave-safe bowl. Heat in 20-second intervals until completely smooth.

Add granulated sugar to the chocolate mixture and whisk until fully combined. Crack in six large eggs, whisking until smooth and glossy.

Gently fold in cocoa powder and all-purpose flour until no streaks remain, then add both white and dark chocolate chips, folding them in evenly.

Pour the brownie batter into your prepared baking tin and bake for approximately 30 minutes until a toothpick inserted comes out with a few moist crumbs attached.

Let the brownies cool at room temperature for about 1 hour, then transfer to the refrigerator for an additional hour.

Cut 12 brownie squares from your cooled brownies. For Frankenstein decorations, pipe green frosting around the edges and create features using a piping bag.

For the mummy brownies, pipe light yellow frosting lines across the surface and add black frosting for spooky eyes.

For spider monster brownies, pipe purple frosting around edges and create features using black frosting and liquorice.

For Dracula brownies, pipe black and white frosting for a widow's peak hairstyle, adding eyes for decoration.