Pre-heat the oven to 180 degrees C/360 degrees F/Gas mark 4_.
Prepare two 20cm (8inch) sandwich tins by greasing them thoroughly and lining the base with non-stick parchment.
Place the butter in a mixing bowl and beat it until it is pale and light. Add the sugar and continue beating until it's light and fluffy. Break the eggs into a separate bowl and beat them lightly.
Add the eggs a spoon at a time to the batter mix, beating in each addition thoroughly before adding the next. This is so the mix won’t split.
Sift the self raising flour, cocoa powder and salt together. Use a large spoon, fold in the sieved flour to the egg and butter mixture. The mix should now be ready, however if it looks a bit thick, add in a drop of room temperature milk to loosen it
Divide the mixture equally between the two prepared tins. Tap the tins down gently to expel any large air pockets and level the cakes. Place into the preheated oven for 25-30 minutes.
When ready, cool on a cooling wire rack and remove the baking parchment.
For the ganache:
Coarsely chop up the dark chocolate on a clean chopping board and place in a heatproof mixing bowl. Over a medium heat on the hob, heat the double cream in a saucepan until it starts to boil. Once it starts bubbling remove it from the heat and carefully pour it over the chocolate in the bowl. Stir until all the chocolate has melted and you are left with a glossy ganache.
For the buttercream:
Gently soften the butter and start to beat in a mixing bowl until light and fluffy. Add in the vanilla and mix thoroughly. Sift in the icing sugar and continue to beat well until the mix is pale and smooth.
Remove the Oreos from the pack and set 3 or 4 aside for decoration. Remove the centres from the other Oreos and bash them up in a sandwich bag using a rolling pin. Pour the crumbed Oreos into the buttercream and mix through using a wooden spoon.