Step 1

Preheat oven to 350°F. Lightly grease 1 rimmed (18x13-inch) baking sheet with cooking spray, then line with parchment paper. Or, alternatively, lightly grease 2 (9x13-inch) rimmed baking sheets with cooking spray and line with parchment paper. Prepare cake mix according to recipe instructions. Fold 2/3 cup sprinkles into batter if using, then spread batter into prepared baking sheets. Bake in preheated oven 20-23 minutes, rotating halfway through, or until a toothpick inserted in the center of the cake comes out clean.

Step 2

Let cake cool 10 minutes in pan, then invert onto a wire rack to cool completely, about 30 minutes. Meanwhile, prepare Cream Cheese Frosting and place in a piping bag. If all frosting does not fit in one bag, do this in batches. Place cooled cake on a cutting board. Use a mason jar to cut 18 circles of cake; set aside.

Step 3

Assemble the Jars: Place one circle of cake in a mason jar, lightly pressing cake to very bottom of jar. Pipe frosting about 1/2-inch thick on top of cake layer, then top with a second cake layer. Pipe another 1/2-inch of frosting on top of second layer, then add one final layer of cake and a final 1/2-inch layer of frosting. Sprinkle with sprinkles, if desired. Place lid on mason jar and repeat with remaining cake, frosting, and sprinkles.