Step 1: Preheat your oven to 350°F (175°C) and prepare the blue velvet cake according to the package instructions using the eggs, oil, and water.
Step 2: Once the cake is baked and completely cooled, crumble it into small bite-sized pieces in a large bowl.
Step 3: In a separate bowl, cream the softened butter until smooth. Gradually add powdered sugar, vanilla, and blue food coloring, beating until light and fluffy.
Step 4: Place a handful of cake crumbles into the bottom of each glass jar to create the first layer.
Step 5: Pipe or spoon a layer of blue frosting over the cake, followed by a sprinkle of mini chocolate chip cookies.
Step 6: Repeat the layers of cake, frosting, and cookies until the jar is filled to the top.
Step 7: Finish each jar with a final swirl of frosting, two candy eyes, and a cookie half to resemble the Cookie Monster.