12 black cupcake liners; 5 piping bags fitted with small star tips
For the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin tin with black cupcake liners.
Mix the flour, baking soda and salt in a medium bowl. Microwave the milk in a large microwave-safe bowl until bubbling, about 1 minute. Whisk in the cocoa until dissolved. Whisk in the granulated sugar, oil, vanilla and egg until smooth. Fold in the flour mixture until combined.
Divide the batter among the cupcake liners (about 1/4 cup per liner). Bake until a toothpick inserted in the center comes out clean, 15 to 18 minutes. Let the cupcakes cool in the pan for 10 minutes, then remove to a wire rack to cool completely, about 1 hour.
For the frosting: Meanwhile, beat the butter in an electric mixer fitted with a paddle attachment on medium-high speed until light, fluffy and almost double in size, 4 to 5 minutes. Turn the mixer to low, add the marshmallow creme and confectioners' sugar and beat until just combined, about 1 minute.
Divide the frosting into 5 separate bowls. Color one bowl with purple gel food coloring, one bowl with orange gel food coloring, one bowl with neon green gel food coloring, one bowl with blue gel food coloring and one with black. Spoon into 5 separate piping bags fitted with small star tips (we used a size 15 tip).
For the decorations: Pipe monster faces and hair on top of the chocolate-covered graham cookies. Decorate with candy eyes. Use the remaining frosting to pipe on top of the cupcakes. Place one decorated cookie on top of each cupcake.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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