Preheat oven to 180C. Grease and line a Swiss roll 32cm x 26cm tin with baking paper.
Sift flour and cocoa onto a sheet of baking paper. Using an electric beater, beat eggs, egg yolk and sugar for 10 minutes or until thick and creamy. Add flours and cocoa. Gently fold into egg mixture. Add water and gently stir into mixture. Do not stir too heavily as this will deflate sponge mixture.
Pour mixture into prepared pan. Bake for 12 minutes or until sponge springs back when lightly touched.
Lay a clean tea towel onto a flat surface. Top with a sheet of baking paper. Sprinkle extra sugar onto baking paper. Turn sponge onto caster sugar.
Gently peel baking paper from sponge. Roll sponge from long side to long side, using tea towel as a guide. Put onto a wire rack and cool.
Using an electric hand beater, beat cream until soft peaks form. Add peppermint essence and green food colouring, 1 drop at a time until desired colour.
Unroll cold sponge. Spread 2/3 cream onto cake. Top with half the chopped peppermint crisp. Roll sponge from long edge to long edge and place onto a serving platter.
Put into a small pipping bag. Pipe cream down the centre of roulade. Sprinkle with remaining peppermint crisp and decorate with small mint leaves. Serve.