1. Preheat oven to 170°C. Line eighteen 80ml (1/3 cup) capacity muffin pans with paper cases.
2. Sift flour, cocoa powder and bicarbonate of soda into a bowl. Stir through the sugar.
3. Whisk the buttermilk, butter, eggs, vinegar and vanilla in a large jug until combined. Make a well in the centre of the flour mixture.
4. Add the buttermilk mixture. Stir until just combined. Stir in the food colouring.
5. Set aside some crushed Oreos for garnish, then add remaining Oreos to the cake mix and stir to combine.
6. Divide the mixture among the lined pans. Bake for 20-25 minutes or until a skewer inserted into the centres comes out clean.
7. Transfer to a wire rack to cool completely. Pipe or spread over the Philly frosting and serve.