Preheat your oven to 200C/180 fan and grease and line a swiss roll tin (32x22cm) with parchment paper.
Add the eggs and sugar to a bowl, and whisk for about 5 minutes on a medium-high speed, until the mixture is paler, and mousse like.
Sift the self raising flour and cocoa powder into the bowl, and add the peppermint extract.
Carefully fold the ingredients together in the bowl until the mixture is combined.
Pour the mixture into the swiss roll tin and smooth over.
Bake in the oven for 10 minutes.
Once baked, remove from the oven. Prep another piece of parchment paper, and dust with icing sugar.
Flip the swiss roll onto the sugar covered piece of parchment paper. Carefully remove the original piece of baking parchment that was in the tin.
Carefully fold the sponge up from short side to short side quite tightly (be careful, it will be very hot!) and leave to cool fully all rolled up.
Buttercream
Beat the unsalted butter on it's own for a few minutes to loosen.
Add the icing sugar and beat again - it will be quite thick.
Add the double cream, and peppermint extract, and beat again until smooth!
If using, add the green food colouring now.
Ganache
Add the dark chocolate and milk chocolate to a bowl.
Heat the double cream in a small pan until just before boiling point.
Pour the hot cream over the chocolate and leave to sit for at least 5 minutes. Stir to combine until smooth. If chocolate lumps still remain, add back into the pan on a low heat and stir until smooth, or microwave briefly.
Let the ganache sit in the fridge for 30-45 minutes, and then whip with an electric whisk until smooth and fluffy! Sometimes this can take longer than this though to get to the whippable stage.
Decoration
Carefully unroll the sponge - PLEASE be as careful as possible so you don't break the sponge. Don't worry too much if it does though as you can stick it back together.
Spread the buttercream onto the unrolled sponge, and re-roll into the classic swiss roll shape.
Slather the ganache over the outside of the cake, and use a fork/cake skewer to create a bark look! Or, pipe the ganache on instead.
Lightly dust with icing sugar, and sprinkle with crushed mint chocolates or candy canes if you fancy - but these are optional.
If you don't want to use ganache you can use a buttercream instead! Double the buttercream recipe, and split into two. Add 25g cocoa powder to one half.