Preheat the oven to 180C/350F and grease and line two 9 inch round pans.
Place dark chocolate into a heatproof bowl and place in the microwave in 20 second increments, stirring in between until melted. Alternatively, you can place the bowl over a simmering pan of water on a low heat on the hob and stir until melted. Leave to one side.
Place oil, eggs, vanilla extract and sugars into a large bowl or the bowl of your stand mixer and beat on medium-high speed until light and frothy, about 1 minute.
Add in the melted chocolate and mix until well combined.
Add in flour, cocoa powder, baking powder, bicarbonate of soda, salt and sour cream and mix on low until well combined and smooth.
Divide cake batter between your lined cake tins. I use a 1/2 cup to divide my batter and ensure they're both even.
Smooth and level the batter in each tin and then take one of your cake layers and place whole Oreo's all over it - I started with a circle on the outside, then worked my way in to the centre - this was 18 and 1/2 Oreo's for me!
Place in the oven; the cake not covered in Oreo’s will bake faster – 18-23 minutes. Remove that from the oven when risen and an inserted skewer in the centre comes out clean.
The cake with Oreo’s on the top will take 20-25 minutes – remove once risen and an inserted skewer into the centre comes out clean.
Leave to cool in the pans for at least an hour before transferring to a wire rack to cool.
Once cooled, take the cake with Oreo’s on the top and place it onto a plate, Oreo side facing up.
Then, make the filling: Place double cream, vanilla extract and icing sugar into a large bowl and whisk until thick and it holds a peak when you pull the whisk out.
Divide the mixture in half, placing one half into another bowl.
Place one full sleeve of Oreo’s into a large Ziploc bag and beat with a rolling pin until you have small crumbs. Alternatively, you can place them into your food processor and blitz until you have crumbs.
Tip the Oreo crumbs into one of the bowls of whipped cream. Fold in slowly, being careful not to whip anymore air into the mixture otherwise you may over whip your cream. Leave to one side.
Take the bowl with plain double cream and spoon it onto the cake with a layer of Oreo’s. Smooth until level with a spatula or wooden spoon.
Take the plain chocolate cake and place it bottom side up, on top of the whipped cream topped cake.
Next, spoon your Oreo cream mixture onto the cake and smooth with a spatula until level. If you want, reserve a little mixture to pipe little mounds around the edge, this is optional.
If you do pipe around the edge (I used a large open star nozzle) then cut 4 Oreo’s in half and place in the centre of each mound of cream. Crumble the last remaining Oreo into the centre of the cake, optional.
Cake will keep covered, in the fridge for 2 days. Best served at room temperature.