In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until evenly combined.

Add the egg, whole milk, vegetable oil, and vanilla extract. Whisk until smooth.

Slowly pour in the hot water and stir until combined. The batter will be thin, which helps create soft chocolate cupcakes.

Divide the batter evenly among the prepared cupcake liners, filling each about two-thirds full.

Bake for 18–20 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Allow the cupcakes to cool in the pan for 5 minutes. Then transfer them to a wire rack and let them cool completely.

Beat the softened butter with an electric mixer for about 3 minutes. Gradually add the powdered sugar, then mix in the vanilla extract, salt, and 2 tablespoons of heavy cream. Beat until light and fluffy, adding a little more cream if needed.

Divide the buttercream into three bowls. Add orange gel coloring to the first, purple to the second, and green to the third. Mix each portion until the color is even.

Transfer the colored buttercream to piping bags fitted with star or grass tips. Pipe colorful swirls, spikes, or fuzzy monster textures onto the cooled cupcakes.

Gently press 2–3 candy eyes into each cupcake. Add Halloween sprinkles or mini chocolate candies if desired. Serve and enjoy!