Preheat the oven to 180C (350 F) about 10 minutes before you want to bake your roulade.
Lightly oil and line a 33x23 cm tin with nonstick baking parchment. If you have swiss roll tin, use it, but if not, use a large shallow tray.
Break the chocolate into small pieces and place into a heatproof bowl set over a saucepan of simmering water. Leave until almost melted, stirring occasionally. Remove from the heat and leave to stand for about 5 minutes to cool down.
Whist the egg yolks with the sugar until pale and creamy and the whisk leaves a trail in the mixture when lifted, then carefully fold in the melted chocolate.
In the clean grease free bowl, whisk the egg whites until stiff, then fold one large spoonful into the chocolate mixture.
Mix lightly, then gently fold in the remaining egg whites. Pour the mixture into the prepared tin and level the surface. Bake in the preheated oven for 20-25 min or until firm.
Remove the cake from the oven, leave in the tin and cover with a wire rack and a damp tea towel. Leave for 8 hours or preferably overnight. Leaving the cake in the tin overnight gives it a fudgy texture and also means that the cake is less likely to break when it is rolled up.
Next day, dust a large sheet of nonstick baking parchment with two tablespoons of icing sugar. Unwrap the cake and turn carefully onto the greasepreoof paper. Remove the baking parchment.
Whip the cream with the liqueur until soft peaks form. Spread over the cake, leaving about 2 cm border all round.
Using the paper to help, roll the cake up from the short end. Transfer to a serving plate, seam side down and dust with the remaining icing sugar. Decorate with fresh raspberries or cherries and mint.