Preheat the oven according to the cake mix box. Line 2 standard muffin pans with paper liners.
In a large bowl, prepare the yellow cake mix following the package directions, adding the eggs, oil and water called for on the box.
Divide the batter among the liners, filling each about 2/3 full. Bake as directed on the box until tops spring back lightly or a toothpick comes out clean.
Let cupcakes cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely. If they are even slightly warm, the frosting will slide off.
Beat the softened butter, in a large (like large!) mixing bowl until smooth and creamy. Add 4½ cups powdered sugar, vanilla and a pinch of salt. Mix on low until it comes together, then increase speed until smooth and thick.
Add heavy cream or milk 1 tablespoon at a time on low speed until the frosting is thick and easy to pipe. Adjust with more powdered sugar if it is too soft or more cream if it is too stiff.
Divide frosting into bowls and add gel food coloring, mixing until the color is even.
Fit a piping bag with a large round or open star tip and fill with colored frosting.
Pipe a tall mound or swirl on each cupcake, building height so there is plenty of frosting to stick decorations to.
Press candy eyes into the frosting. Gently press candy corn into the front of the frosting mound with pointed ends facing the center to form sharp-looking teeth.
Place decorated cupcakes in a single layer in a covered container. Store at room temperature up to 1 day or refrigerate up to 3 days. If chilled, let cupcakes sit 15 to 20 minutes at room temperature before serving so the frosting softens.