Preheat the oven to the temperature indicated on the box of cake mix. Lightly grease a 9×13-inch baking pan with cooking spray, then line it with parchment paper leaving an overhang on the sides to help remove the cake once it is baked.

Mix the chocolate cake batter according to the instructions indicated on the box, then pour the finished batter into the lined baking pan. Bake according to the box directions until a tester inserted into the center comes out clean, approximately 30-35 minutes. 

Place the baking pan on a wire rack and let the cake cool completely in the pan before proceeding, about 1-2 hours. 

Once the cake is cool, make the buttercream. Add the butter to the bowl of a stand mixer fitted with the paddle attachment. Beat the butter on medium-low to medium speed until creamy, about 2 minutes.

Scrape down the sides and bottom of the bowl and paddle then add the vanilla and your desired amount of gel food coloring. I went with a heaped teaspoon. With the mixer on low, slowly begin to incorporate the powdered sugar. Once all of the powdered sugar has been added, scrape down the sides of the bowl and paddle then add 3 tablespoons of milk. Beat on medium-low speed, gradually working up to medium high as the milk incorporates into the frosting. Beat for 1-2 minutes on medium high until the buttercream is thick and creamy. If it seems a bit too thick, add another tablespoon of milk. 

Use the parchment paper to lift and remove the cake from the pan then carefully flip it over onto your work surface or a large cutting board. Using a round cookie cutter that is the size of the canning jars, cut out twelve rounds of cake. Use an offset spatula to help lift each round of cake, then place the flattest side down into each jar. 

Fill a large piping bag or gallon ziptop bag with buttercream. If you’re using a ziptop bag, snip off one of the corners. Pipe the buttercream onto the top of the cake in each of the jars, making sure to overfill them past the top. Then, go in with an offset spatula and level out the surface of the buttercream so that it is even with the top of the jar. 

Empty the sprinkles onto a small plate or wide, shallow bowl. Dip the surface of the buttercream into the sprinkles to coat, pressing firmly. Repeat with the remaining cakes. 

Serve right away, or carefully put the lid onto each jar for storing or transporting. The cakes can be stored with their lids secured at room temperature for up to 3-4 days. After 4 days, I recommend moving leftovers to the fridge. Enjoy them within the next couple of days.