Line the bottoms of three 8 inch cake pans with parchment paper and grease the sides. Preheat oven to 350 degree.
Mix together the oil, sugar and vanilla extract. Add eggs and mix until well combined.
Combine flour, cocoa, baking powder and salt. Slowly add to the egg mixture until well combined.
Pour the batter evenly into the three pans and spread evenly. Bake for 25-30 minutes, or until a toothpick comes out with a few crumbs.
Remove brownies from oven and allow to cool for about 10 minutes in the pan, then remove to cool on wire rack. While brownies cool, make cookie dough.
Cookie Dough
Cream butter and sugars together until light and fluffy, about 3-4 minutes. Mix in peanut butter and vanilla extract.
With the mixer on low speed, add flour. Add milk to preferred thickness. Stir in chocolate chips. Dough will be thick. Set aside.
Assemble Cake
Once the brownies are cool, it’s time to put the layers together. You can pile the cookie dough onto the brownies, stack the layers and leave messy layers, or use the method I used for having clean edges/sides of the cake.
Place half of chocolate chips in a metal bowl. Microwave half of the heavy cream until it starts to boil. Remove from microwave and pour over chocolate chips.
Cover bowl with saran wrap for 5-7 minutes. Whisk chocolate and cream until smooth.
Using one of the same 8 inch cake pans that you baked the brownies in, put parchment paper or saran wrap in the bottom and up the sides of the pan. You will use it to lift the layers out of the pan.
Place the first brownie into the bottom of the pan. Add about 3 tbsp of chocolate ganache on top of the brownie and spread into a thin layer. This will help the cookie dough stick to the brownie.
Put half of the cookie into the pan and spread evenly on top of the brownie. Remove the first two layers from the cake pan and place on it’s serving dish. Repeat steps 5-8 with the second brownie and remaining cookie dough. Then remove from pan.
Put another 3 tbsp of chocolate ganache on top of the cookie dough layer from the first section of cake and spread thinly. Place the second section of cake onto the first.
Put another 3 tbsp of chocolate ganache on top of the cookie dough layer on top (the fourth layer) and spread thinly. Top with remaining brownie.
Finish Cake
Place remaining chocolate chips in a metal bowl. Microwave remaining heavy cream until it starts to boil. Remove from microwave and pour over chocolate chips.
Cover bowl with saran wrap for 5-7 minutes. Whisk chocolate and cream until smooth.
Allow to sit for a few minutes until it just starts to thicken, then pour over the top of the cake.
Place cake in refrigerator and make the peanut butter icing. To make the icing, beat the shortening and butter together until smooth.
Add the peanut butter and mix until smooth. Slowly add the powdered sugar and mix until smooth. Add water or milk.
Remove cake from fridge, and pipe icing onto the top edge of the cake and sprinkle with mini chocolate chips, if desired.