1. Preheat oven to 350 degrees. Place parchment paper in the bottom of a 9 inch cake pan and grease the sides.
2. Cream butter and sugars together until light and fluffy, about 3-4 minutes.
3. Mix in egg and vanilla extract.
4. With the mixer on low speed, add flour, baking soda and salt.
5. Stir in chocolate, peanut butter and white chocolate chips. Dough will be thick.
6. Press the dough evenly into the cake pan.
7. Bake for 18-20 minutes or until the edges are slightly golden. The center will look undercooked, but it will continue cooking and firm up as it cools.
8. Remove from oven and allow to cool completely in the cake pan, then transfer to serving plate.
9. To make the frosting, beat the shortening and butter until smooth.
10. Slowly add 1 cup of powdered sugar. Mix until combined.
11. Mix in the cocoa and 1 tbsp water.
12. Add the rest of the powdered sugar and mix until smooth. Add more water to get the right consistency.
13. Pipe the icing onto the edges of the cookie cake, then store in an airtight container until ready to serve.
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