Preheat your oven to 350°F (175°C). Line a 12-cup muffin tin with cupcake liners.
In a large bowl, whisk together the flour, cocoa powder, baking soda, salt, and granulated sugar. Add the vegetable oil, water, vinegar, and vanilla extract. Stir until just combined (do not overmix).
Divide the batter evenly among the cupcake liners, filling each about two-thirds full. Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool completely on a wire rack.
For the frosting: In a medium bowl, beat the softened butter with an electric mixer until creamy. Gradually add the powdered sugar, 1 cup at a time, beating well after each addition. Add 2-3 tablespoons of heavy cream and beat until light and fluffy. Divide the frosting into separate bowls and tint each with different food colors (e.g., purple, green, orange).
Once the cupcakes are cool, pipe or spread the colored frosting onto each cupcake using a piping bag with a star tip or a spatula for a textured “monster” look. Place candy eyes on top (use 1, 2, or 3 eyes per cupcake for variety). Add sprinkles or other edible decorations if desired.
Arrange the decorated monster cupcakes on a platter and serve. Enjoy your spooky, fun treat!