Add the chocolate cake mix, oil, and eggs to a mixing bowl and beat until batter forms. (You can also mix by hand.) Fold in the chocolate chips
Using a 2-tablespoon spring release scoop, spoon the dough onto a baking sheet lined with a silicone mat. With a flat-bottom glass, press the cookie dough into a 2-inch circle. Use a rubber spatula to round the edges if you want perfectly round cookies.
Bake at 350°F for 9 minutes. (If your cookies are larger, smaller, you use a dark pan, or your oven hasn't been calibrated your cookies may require more or less cooking time. Watch them closely to get to the degree of doneness you like.
Remove from the oven and allow the cookies to cool on the pan they were baked on for 2 minutes. (This helps them set before moving them.) Then transfer them to a cooling rack to cool before frosting.
Frosting
Cream the butter and shortening with the whisk attachment of an electric mixer until light and fluffy.
With the mixer on low add the powdered sugar. Mix until incorporated.
Add 1 tablespoon whipping cream and vanilla extract and mix. Add the other tablespoon of whipping cream if needed. You want the mixture to be soft and fluffy.
Fold in the crushed Oreo by hand until evenly incorporated.
Spread frosting carefully and evenly over the cookies with an offset spatula or piping bag and a large tip. Serve.
Store leftovers in an airtight container on the counter. They'll stay fresh for 4 or 5 days in an airtight container.