Pre heat oven to 180 degrees / 350 F
Separate the creams from the Oreo biscuits. Crush the Oreo biscuits into a fine flour using a food processor or rolling pin. Add baking powder to the biscuit flour and mix.
Add biscuit cream and milk to the dry ingredients and mix with a food processor or whisk until combined.
Pour mixture into a baking paper lined tray. Bake for 20 minutes.
Remove from the oven and let cool for 10 minutes. Crumble the cake and transfer back to the same baking paper lined tray or transfer to a smaller one (also baking paper lined).
Push the cake into the tray with your fingers. Compressing the cake as much as you can. This will make it a dense chocolately slice opposed to a dry cake.
Refrigerate for 20 minutes.
Remove and cover with the butter cream frosting.
Take 1 Oreo biscuit (without cream) and crumble on top of the cake.