Preheat oven to 350F. Coat five 6″ cake pans with cooking spray.
In the bowl of a standing mixer, add flour, sugar, cocoa powder, baking powder, baking soda, salt and espresso powder. Whisk together by hand or with the paddle attachment until combined.
Add buttermilk, canola oil, egg and vanilla to flour mixture. Mix on medium speed until fully incorporated. Reduce speed to low ad slowly pour in the boiling water. Continue mixing until combined.
Pour batter into prepared cake pans, about 3/4 of the way full. Bake for about 20 minutes, until a toothpick inserted into the middle comes out clean. Remove and let cool.
For the buttercream and chocolate drips
In the bowl of a standing mixer fitted with the paddle attachment, beat the butter until smooth and creamy, about 1 – 2 minutes. Alternate adding the powdered sugar and heavy cream until combined. Add vanilla and salt and beat for another minute on medium-high. Add crushed Oreos and mix on low until evenly distributed.
To assemble the cake, place one layer on a cake stand then frost. Continue until all layers are used, then frost the edges until smooth. Chill in the freezer for about 10 minutes.
While the cake is chilling, bring the heavy cream to a simmer in a small saucepan over medium heat. Remove from heat and add chocolate. Let sit for about 5 minutes, then whisk until smooth. Allow ganache to cool and thicken for about 30 minutes. Remove cake from freezer and use a spoon to create deliberate drips along the top edges. Then create a smooth layer on top and let harden. If you have leftover buttercream, transfer to a piping bag fitted with your favorite tip (I used Wilton 1M) to decorate the top.