Prepare three 8-inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C).

In a large mixing bowl, cream the butter and sugars together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.

Add sour cream and vanilla extract and mix until well combined.

Add the eggs one at a time, mixing until just combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.

Combine dry ingredients in a separate bowl, then combine the milk and water in a small measuring cup.

Add half of the dry ingredients to the batter and mix until well combined. Add the milk mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated. Stir in the mini chocolate chips.

Divide the batter evenly between the cakes pans and bake for about 21-23 minutes, or until a toothpick comes out with a few crumbs.

Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.

In a large mixer bowl, beat the butter and shortening together until smooth.

Slowly add half of the powdered sugar and mix until smooth.

Add the vanilla extract and 5-6 tablespoons of water or milk and mix until smooth.

Slowly add the remaining powdered sugar and mix until smooth. Add additional water or milk, as needed.

To put the cake together, use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. Place the first cake on a serving plate or a cardboard cake round.

Pipe a dam of frosting around the edge of the cake.

Put the milk for filling into a small bowl. Dip several chocolate chip cookies into the milk, then crumble them up and spread them evenly over the cake layer, inside the dam. You want a full layer of cookie crumbles.

Add a thin layer of frosting over top of the cookies. I used a piping bag and tip to make it easier to add the frosting and spread it evenly.

Add the second layer of cake, then repeat another dam of frosting with the cookie filling and vanilla frosting on top.

Top the cake with the remaining layer and frost the outside of the cake. Refer to my tutorial for frosting a smooth cake, if needed.

Press more mini chocolate chips into the outside of the cake around the bottom. You might need to put a little frosting on the back of the cookies to help them stick.

Use the remaining buttercream to pipe swirls around the top of the cake.

Top the swirls with chocolate chip cookies and add some additional mini chocolate chips, if desired. Store cake in the fridge, but serve at room temperature. Cake is best for 2-3 days.