Preheat the oven to 180°C, fan 160°C, gas 4. Line a 33cm x 23cm Swiss-roll tin with baking parchment. Melt the chocolate in a heatproof bowl over a pan of simmering water (make sure the bowl doesn’t touch).
In a separate bowl, whisk the sugar and egg yolks using a hand-held electric whisk until very thick. Fold in the melted chocolate.
In another bowl, whisk the egg whites until stiff; beat 2 tablespoon into the chocolate mixture, then gently fold in the remainder. Pour into the prepared tin and bake for 15-20 mins until risen and firm to the touch.
Lay out a large piece of baking parchment and sift half the icing sugar over it. Turn the warm roulade out onto the paper and cover with a clean, damp tea towel. Leave to cool.
Meanwhile, whip the cream to soft peaks in a bowl. Peel the top baking parchment layer from the roulade and spread with the cream, then scatter over the cherries. Using the bottom piece of baking parchment to lift the cake, roll up the roulade from one short end.
Transfer to a serving plate, then dust with the cocoa powder and the remaining icing sugar to create a snowy effect.