Preheat the oven to 180 (360 F). Grease and line a round springform cake tin. In a large mixing bowl, beat the butter and sugar with an electric mixer until combined. Add the vanilla and the egg and continue to beat until soft and creamy. Sift in the flour, baking soda and cornflour. Gently fold until dough starts to come together.

Throw in the chocolate chips and fold through. Place all the dough into your prepared tin and press down and spread out as even as possible. Place in the oven for 18 - 20 minutes or until golden brown. Leave to cool completely. Repeat this step to make two cookie layers.

Grease and line a round springform cake tin. In a large mixing bowl, sift the flour, cocoa powder, baking soda, baking powder and then add the sugars. Pop the butter into the microwave for short bursts, 10 seconds or so at a time, until it is just melted. Give it a stir with a fork to eliminate any lumps. Whisk together the butter, eggs, vanilla and buttermilk until combined - don't worry if it goes a little lumpy. Then add the wet mixture into the dry mixture and gently fold until just combined.

Spoon the mixture into the prepared tin and pop into the oven. Bake for 25 minutes or until cooked through. Leave to cool for 10 minutes. Gently run a knife around the edge of the cake before turning out of the tin. To get a nice flat cake which will make it easier to frost, set the cake upside down on a wire rack to cool completely - the domed top will naturally flatten as it cools. Repeat this step to make two cake layers.

In a large mixing bowl, beat the butter until creamy and smooth and then sift in half the icing sugar. Add a tablespoon or two of milk to help loosen up the mixture. Add the rest of the icing sugar and the cocoa powder and continue to beat. Add a tablespoon of milk as needed. The icing should be nice and creamy but thick enough to hold its shape.

To assemble the cake, pop a dollop of frosting onto your plate or cake stand and then place one cookie layer on top. Press down gently. Cover the cookie with a thin layer of frosting going right to the edge, then place a chocolate cake layer on top. Repeat and add the second cookie layer. When it comes to adding the final chocolate cake layer, place it upside down, so the nice flat bottom is at the top. Then gently frost the entire outside of the cake using a spatula.