Add purple food coloring one drop at a time until you reach the color purple you would like. Frost each cupcake with approximately 2 Tbsp of buttercream per cupcake.
Place a large candy eye onto each cupcake.
Fill a small bowl or cupcake liner with purple jimmie sprinkles. Carefully dip each cupcake into the bowl. Cover as much of the cupcake as you can with the sprinkles.
Press a green candy corn into the cupcake above the eye to make a horn. Repeat for the remaining cupcakes.
You could easily make these plain monster cupcakes by switching up the colors of the frosting and the sprinkles. You could do green, blue, red, orange, and yellow monsters very easily. Vary up the look of the monsters by giving some monsters 2 or more eyes as well.