Preheat the oven to 350F (175C ) degrees and line up 3 (6-inch, 15cm) cake tins with parchment paper.
In a bowl of a stand mixer, beat butter until light and fluffy.
Beat the dry ingredients into the butter until well incorporated, scrape the bowl at least once.
Beat the wet ingredients together in a separate bowl.
Add the wet to the butter mixture and mix until just combined. Fold in the crushed gluten -free Oreos at the very end and mix until just combined.
Divide the mixture evenly into the cake pans. Smooth the batter evenly into each pan with the back of a spoon.
Bake at 350F (175C) degrees for about 25-30 minutes or until a toothpick inserted into the middle comes out with only a few crumbs attached.
Allow cakes to cool in the pan for 10 minutes and turn out onto a wire rack to cool completely before frosting.
To make the Buttercream
Using a handheld or stand mixer fitted with a whisk attachment beat the softened butter on medium speed for 2 minutes until completely smooth and creamy. Add confectioners’ sugar, heavy cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 4 minutes. Mix in the crushed Oreos into the large batch of buttercream.
Add up to ¼ cup more confectioners’ sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick.
To Make the Chocolate Ganache
Put the dark chocolate in a bowl and heat the cream in a small pan until just steaming. Pour the cream over the chocolate and leave to melt for 5-10 mins. Stir to make a glossy ganache, then set aside to firm up a little – you want the ganache to be pourable but not too runny, to make drizzly droplets down the side of the cake.
To Assable The Cake
Place one cake layer on your cake stand. Spread a generous, about ¾ cup, frosting over top of the first layer, then pour a little bit of the chocolate ganache over the frosting. (Don't go overboard with the ganache as it will overpower the other flavors.)
Put the next sponge on top and repeat the same step. Then put the last sponge on top and spread the remaining frosting all over the top and the sides of the cake.Reserve about a cup of the buttercream for the dollops.
When the ganache is the correct consistency, spoon the ganache over the top edge of the cake. Fill in the middle of the top of the cake with chocolate too.
To Decorate
Optionally, make pipes on top of the cake with the remaining buttercream frosting and decorate with the crushed gluten free oreo cookies.
The cake is best served within 2-3 hrs of assembling, but will still taste delicious for up to 3 days. Store leftovers in the fridge and allow to come back to room temperature before serving.