Preheat your oven to 350°F (175°C). In a large bowl, combine cake mix with eggs, oil, and water as per package instructions. Mix until smooth, then gently fold in 5 chopped Oreos. Pour into a greased cake pan and bake for 25-30 minutes, or until a toothpick comes out clean.

Remove the cake from the oven and let it cool in the pan for about 10 minutes. Transfer it to a wire rack to cool completely for at least 30 minutes.

In a mixing bowl, beat together the unsalted butter and vegetable shortening until smooth and creamy, for about 2-3 minutes. Gradually add in the powdered sugar, then pour in the milk, vanilla extract, and food coloring, mixing until airy and spreadable.

Crumble the cooled cake into small pieces. Layer in mason jars starting with crumbled cake, then frosting, followed by chopped cookies, repeating the layers and finishing with frosting on top.

Decorate by placing candy eyes on top of the frosting, and add a half chocolate chip cookie below to resemble Cookie Monster's mouth.

Chill the assembled jars in the refrigerator for at least 30 minutes before serving to meld flavors and make serving easier.