Mix cake batter according to the box directions. Place flat-bottomed ice cream cones into the slots of a mini-muffin tin, then tuck aluminum foil around each one so they dont wobble. Drop a tablespoon of cake batter into the bottom of each cone, then use a skewer or the back of a spoon to nestle it in and prevent air bubbles. Fill the cones with more batter, leaving about a 1/2-inch border at the tops. Bake according to package directions until a skewer poked into the center comes out clean, about 30 minutes. Let cool, then pipe or dollop frosting on top and finish with sprinkles.