Line a 24x35cm Swiss roll tin with paper and preheat oven to 180°C.
Melt the dark chocolate in a heat-proof bowl over simmering water.
Whisk egg yolks and sugar until pale and thickened, then stir in cocoa powder.
Whisk egg whites in a separate bowl until stiff peaks form.
Stir the melted chocolate into the egg yolk mixture until fully incorporated.
Gently fold the stiff egg whites into the chocolate mixture.
Pour mixture into the tin and tilt to fill the corners.
Bake for 18 to 20 minutes until the sponge is firm and springy.
Turn the cake onto sugar-dusted paper and remove the lining paper.
Cover with a damp towel and leave to cool completely.
Beat icing sugar and butter, then add peppermint oil and milk to create buttercream.
Spread filling over the cake leaving a 1.5cm border and sprinkle with chocolate chips.
Roll the cake tightly from the short end and transfer to a plate.