Heat the oven to 180ºc/160ºfan and line two deep 8"/20cm cake tins with baking parchment – leave to the side.
In a large bowl, beat together the room temperature unsalted butter and light brown soft sugar until light and fluffy.
Add in the self-raising flour, cocoa powder, eggs and mint extract and beat again briefly until combined.
Divide the mixture between the two tins and smooth it over – bake for 50-55 minutes until the cakes spring back when pressed and a skewer should also come out clean.
Once baked, leave the cake to cool in the tin for 10 minutes, and then remove and leave to cool fully on a wire rack.
Buttercream
In a stand mixer, beat the unsalted butter with an electric mixer with the paddle attachment until it is smooth and loose and then beat in the icing sugar 1/3 at a time until its fully combined.
Add in the green food colouring a little at a time till you get your desired colour, and the mint extract.
Beat the buttercream for 4-5 minutes on a medium speed so it starts to get fluffier and lighter. – if its a bit stiff, add in 1tbsp of boiling water at a time until you get a smooth consistency, if required.
Assembly
Once the cakes are cooled, split each cake into two layers.
Put the first layer on the serving plate spread some of the buttercream onto the top of the first layer, add the second cake on top, repeat, and then top again with some of buttercream and then add the final sponge layer.
Cover the sides of the cake with a thin layer of buttercream for a crumb coat. Chill for 30 minutes.
Once chilled, add another layer of butercream around the top and sides of the cake, and smooth around witht a large metal scraper. Chill again for 30 minutes.
Add the dark chocolate and milk chocolate to a jug, and pour over the double cream. Melt on a low heat, stirring often, until smooth.
Gently pour/pipe the ganache around the edge of the cake to cause the drips, and pour the rest onto the top and spread evenly and leave to set briefly.
Once the chocolate has set, with any leftover frosting, pipe some little rosettes onto the top and COVER IN MINT AERO BUBBLES! Enjoy!