Heat oven to 180C/160C fan/gas 4. Grease and line two 20cm cake tins with baking parchment – if your cake tins are quite shallow, line the sides to a depth of at least 5cm. Put 200g flour, 8 tbsp cocoa powder, 2 tsp baking powder, 1 tsp bicarbonate of soda, 280g light brown soft sugar and 1 tsp salt in a bowl and mix well. If there are any lumps in the sugar, squeeze these through your fingers to break them up.
Measure 200ml buttermilk, 100ml coffee, 150ml oil and 2 tsp vanilla in a jug. Add 2 eggs and whisk until smooth. Pour the wet ingredients into the dry and whisk until well combined. Pour the cake mixture evenly into the two tins, and bake for 25-30 mins until risen and a skewer inserted into the centre comes out clean. Cool in the tins for 10 mins, then turn out onto a wire rack, peel off the baking parchment and leave to cool.
Repeat steps 1 and 2, so that you have 4 sponge cakes in total. These sponges can be made up to three days ahead and will stay moist if wrapped in cling film, or you can wrap well and freeze for up to two months.
To make the icing, put the butter in a large bowl and sieve in half the icing sugar. Roughly mash together with a spatula, then whizz with a hand mixer until smooth. Add the cream cheese and sieve in the remaining icing sugar, mash together again, then blend once more with the hand mixer. Put the Oreos in a food processor and whizz to fine crumbs. Add the biscuit crumbs to the icing and mix again until combined.
Next, make the cookie cake toppers. You’ll need one Oreo for each letter of the word you’d like to write – someone’s name or, like we have done, ‘WOW!’ Carefully insert a skewer into the creamy filling in the middle of each Oreo – some cookies may break, so save these ones for later. Melt the chocolate in the microwave, stirring every 20 secs or so, or in a heatproof bowl suspended over a pan of gently simmering water. Spread a little chocolate over the surface of one side of each cookie. Roll out the fondant icing to the thickness of a 50p coin, and cut out your letters, then stick these to the cookies and set aside to dry. Alternatively, let the chocolate dry, then use an icing pen to write the name or word directly on the surface.
To assemble the cake, stick one of your sponges to a cake stand or board with a little of the cream cheese icing. Use roughly half the icing to stack the remaining cakes on top, spreading a generous amount between each layer. Pile the remaining icing on top of the assembled cake, and use a palette knife to ease it over the edges, covering the entire surface of the cake. Tidy the plate with a piece of kitchen paper.
If you have any leftover Oreos, break them into pieces and push these around the base of the cake. Insert the dried cookie cake toppers into the top of the cake and serve. Store leftovers in the fridge for up to 2 days, but bring back to room temperature for an hour or so before eating.