Start by dividing your American buttercream frosting into seven portions. Use gel food colors to create vibrant shades for your monster decorations, blending thoroughly until you achieve your desired hue.

Preheat your oven to 175°C (340°F). Grease and line an 8x8-inch baking tin with parchment paper.

In a microwave-safe bowl, combine unsalted butter and dark chocolate. Microwave in 20-second intervals, stirring frequently, until smooth and glossy.

Stir in the granulated sugar until combined, then beat in the large eggs one at a time. Gradually sift in the unsweetened cocoa powder and all-purpose flour.

Fold in both white and dark chocolate chips until evenly distributed throughout the brownie batter.

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

Allow the brownies to cool at room temperature for approximately 1 hour, then refrigerate for an additional hour.

Gently lift the brownies out of the tin and cut out desired shapes using a cookie cutter or a sharp knife.

Using your prepared frosting, pipe whimsical monster designs onto the brownies and add decorations as desired.