Heat the oven to 180C/fan 160C/gas 4 and line three 20cm loose-bottomed cake tins. Stir together the flour, cocoa and bicarb in a bowl. Melt the chocolate, sugar and butter with 100ml of water in a pan until smooth, then leave to cool.
Once cooled, transfer to a mixing bowl and whisk in the eggs one by one, until combined, followed by the vanilla extract. Slowly whisk the wet ingredients into the dry ingredients and stir until fully combined. Divide equally between the three cake tins and bake for 20-25 minutes or until risen and firm to the touch. Leave to cool completely in their tins.
To make the Oreo buttercream, blitz the Oreos in a food processor to fine crumbs. Tip the butter into a large bowl and, using an electric whisk, beat until really light and fluffy. Add the icing sugar, a spoonful at a time, whisking well in between, until you have a really light buttercream. Add the Oreos and whisk again. Add a little milk to loosen if it is thick.
Put one of the sponges on a cake stand or plate and, using a palette knife, spread over a thick layer of icing, then repeat with the other sponges. Spread a layer of icing over the sides. Push the Oreos into the top of the cake to decorate and cut into slices to serve. Will keep for three days in an airtight container.