Step 1: Preheat oven to 350 degrees.
Step 2: Combine vanilla cake mix, 1 1/4 cup water, 1/2 cup vegetable oil, and 3 eggs into a large bowl. Add purple food coloring and stir until combined.
Step 3: Combine chocolate cake mix, 1 1/4 cup water, 1/2 cup vegetable oil, and 3 eggs into a large bowl.
Step 4: Scoop 1/4 cup of purple cake batter into mason jar. Add 1/4 cup of chocolate cake batter. Continue to alternate layers of purple and chocolate cake batter until mason jar is half full.
Step 5: Fill a baking pan with a thin layer of water. Place cake jar on pan. Bake cake jar at 350 degrees for 25-35 minutes.
Step 6: While cake jar is baking, roll fondant into 1″ balls. Shape into horns.
Step 7: To prepare mint icing, combine unsalted butter, powdered sugar, milk, and peppermint extract into a bowl. Add electric green food coloring.
Step 8: Once cake jars have cooled, pipe icing on top of the cake. Add purple sprinkles, and place two horns to finish the look.