Whisk the flour, black cocoa, baking powder, and salt together in a large bowl.
In a separate bowl, whisk the melted butter, eggs, sugar, and, vanilla together until combined.
Stir in about half of the flour mixture and the brewed espresso. Then stir in the milk and the remaining flour mixture and mix until fully combined.
Evenly spray each cake pan with an oil based pan spray (I don't recommend butter as the butter solids can cause the cake to stick to the pan).
Pour ⅓ of the batter into each cake pan (they will only be about half full).
Bake at 350°F for 25-30 minutes or until a cake tester comes out clean.
Place a cooling rack on top of each cake pan and invert. The pan should slide cleanly away.
Once the cakes are fully cooled. Using a sharp bread knife, trim the cakes until the layers are even. Wrap each layer tightly in plastic wrap and place in the freezer overnight.
Black Cocoa Buttercream
In a large bowl using a handheld or stand mixer fitted with a paddle attachment, cream the room temperature butter and powdered sugar until smooth.
Add in the black cocoa powder and milk until fully combined.
Beat the frosting on high for 4-5 minutes until light and fluffy.
Marshmallow Spiderwebs
All you have to do is place the marshmallows in a microwave safe bowl (I prefer glass) and microwave for 10-20 seconds. Note the marshmallows will be very hot! Stir the marshmallows until smooth and let cool for 1-2 minutes until you have a thick, stringy goop that is no longer hot to touch.
Assembly
Remove cake layers from the freezer. Place the bottom layer of cake on your cake stand and spread about ¼ of the black cocoa buttercream on top and smooth into an even layer.
Then top with the next layer and repeat. Place the final cake layer upside down so that the top of your cake is as even as possible.
Smooth the rest of the buttercream on the top and sides of your cake as desired.
Use two forks to pull strings of the melted marshmallow and drizzle around the sides of the black cocoa cake. (Or wearing disposable vinyl gloves, use your hands to spread the marshmallows around the sides).
This recipe is designed for 3 six inch cake pans. It will also work with 2 eight inch cake pans.
Store the cake in the fridge for up to 4 days. However, it will taste best when served at room temperature! I like to let it sit at room temperature for about an hour before serving.