Preheat oven to 350°F/175°C. Grease 2x 8-inch cake pans with butter, then line the bottoms with parchment paper.
Combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt in the bowl of a stand mixer, using the paddle attachment. Set aside
In a large jug, combine the buttermilk, oil, eggs, and vanilla
While the mixer is on low speed, slowly pour in the ingredients from the large jug
While the mixer is still running, slowly add the hot water and mix. Your batter will be very thin. Make sure the mixer is on your lowest speed so it doesn't splash everywhere
Pour the batter into the prepared pans. Bake for 35-40 minutes, until a toothpick comes out clean when inserted
Allow the cakes to cool completely before removing from pans
Frosting
In a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar together on low speed
Increase the speed to medium-high and beat until it becomes thick
Add the vanilla extract and salt and beat on medium-high
Turn the mixer to low, and pour in the cream until it becomes creamy and spreadable
Fold in the Oreo crumbs
Ganache Drip
Put the chopped chocolate and double cream in a bowl and microwave in 15 second bursts until the chocolate is melted. Keep checking the texture of the ganache until you can drip it down the side of a bowl and it doesn't continue running down.
Assemble
Place one layer down and spread some of the frosting over it. Sprinkle over your chopped oreos. Place the next layer on top and gently press down. Frost the cake (keep some to decorate the top of the cake!) and chill for 10 minutes.
Use a spoon to place the drip over the edge and let it slowly drip down. Repeat all along the cake. If the ganache doesn't drip down, heat it up in the microwave for a few seconds. Spoon over the remaining ganache on top and use a small offset spatula to evenly spread the ganache on top.
Use a Wilton 1M tip to decorate the top of the cake.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your frosting much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Oreos: If you are frosting the top of the cake with a piping tip, make sure to pulse your Oreos into a fine crumb. If they are too big they will clog the piping tip. Buttermilk: No buttermilk on hand? Mix one tablespoon of lemon juice or white wine vinegar with 1 cup of milk, then let it sit for 5 minutes so it can curdle. Storage: Cover the cake tightly and keep in the fridge for up to 5 days. Freezing: Freeze for up to 3 months, let the cake defrost overnight in the fridge and then come to room temperature when you’re ready to eat it.