Preheat oven to 350 degrees F and prepare a half sheet pan with parchment paper
Prepare the cake mix according to the package or recipe instructions
Bake the cake on the prepared half sheet pan (the cake will be thin)
While the cake is baking, prepare the frosting and mix in rainbow sprinkles
Once the cake is baked and out of the oven, allow the cake to cool completely
Using a round cookie cutter, cut out cake rounds that are slightly smaller than the mason jar opening
Layer the cake round at the bottom of the jar, pipe the frosting, add a topping if necessary and repeat with the cake round and finishing with the frosting and sprinkles
Seal the jar with the lid
Wrap a ribbon around the jar and add a birthday tag