Arrange the cupcakes on a baking sheet. Place them in freezer for 15 minutes. Scoop the ice cream into the bowl of an electric mixer and beat with a paddle attachment until it's soft but still holds its shape, about 2 minutes. Spoon the ice cream over the cold cupcakes, creating swoops and peaks. Decorate with the sprinkles or candy, if using, and serve immediately, or return to freezer and chill until the ice cream is firm, about 30 minutes.