Preheat oven to 325° F and prepare two deep 8" cake pans by lightly greasing only the sides with shortening and dusting with all purpose flour. Set aside.
Combine the cookie crumbs and butter until thoroughly combined.
Press into the bottom of the prepared cake pans. Set aside
In the bowl of a stand mixer whisk together bananas, milk, buttermilk, sour cream, vegetable oil, eggs and vanilla until well combined.
Sift in the cake mix. Mix until just combined. The batter should be smooth and completely combined but do not overmix!
Evenly divide batter between your cake pans and bake for 35-40 minutes or until a toothpick comes out clean.
When the cake is done, the top should spring back to the touch and when inserting a toothpick in the center it should come out clean with no wet batter.
Remove from the oven, go around the sides with a small offset spatula to separate the cake from the pan but not removing it from the cake pan just yet.
Allow cakes to cool in the cake pans for about 5 minutes, then carefully flip the pans and remove the layers from the cake pans onto a cooling rack to cool completely before decorating.