Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with cooking spray. Make cake batter as directed on box. With rubber spatula, fold in candy sprinkles. Pour batter into pan.
Bake as directed on box for 13x9-inch pan. Cool completely, about 1 hour.
In small bowl, mix frosting and 1/8 teaspoon green food color until well blended. Spread frosting over cake. To form witch’s hat, insert waffle cone at an angle into top center of cake until rounded edge is level with surface of cake. Tip of cone should be raised about 2 inches directly above top edge of cake pan.
With toothpick make 5 inch long oval outline of witch’s face directly below “hat”. Immediately drop 8 drops green food color randomly over surface of frosting, avoiding “witch’s face”; swirl with knife.
Using ribbon tip, frost “witch’s hat” with black icing; add a “brim” around the base of the “hat” and frost small areas below her face to look like a cape.
Cut and shape gumdrops as desired to look like eyebrows, eyes, nose, mouth, hat buckle and cape button. Cut candy corn into smaller pieces and add to look like teeth. Use drawing tip in black icing to draw witch’s hair. Store loosely covered.