Follow steps on the cake mix box to bake the cake. After mixing the batter, fold in about half of the chopped Oreos. Bake according to box directions and let cool completely for at least 30 minutes.

In a large mixing bowl, cream together the butter and shortening until smooth.

Add in one cup of powdered sugar and the vanilla extract. Mix on low speed until combined, then increase to medium speed.

Add the remaining two cups of powdered sugar, milk (or coffee creamer), and the gel food coloring. Mix starting on low speed and gradually increasing to high. Beat for 2-3 minutes until well combined.

Test the consistency of the frosting. If it's not smooth enough to pipe, add another tablespoon of milk and beat on medium-high for 4-5 more minutes until it's light, fluffy, and easy to pipe.

Fit a piping bag with a large star tip (like a Wilton 1M) and fill it with the blue frosting.

Add 1-2 scoops of the cooled cake into the bottom of each mason jar. Use a spoon to gently press the cake down.

Pipe a layer of frosting over the cake. Sprinkle a layer of chopped Oreos and chocolate chip cookies on top of the frosting. Add another scoop of cake, followed by frosting, and then more cookies.

Repeat the layering process if there is space in the jar, ending with a final layer of frosting.

Decorate the top of each cake jar by placing two candy eyes and a half of a chocolate chip cookie to resemble the Cookie Monster.

Optional: Tie a blue ribbon around the top of each jar for an extra decorative touch.