Heat oven to 220C/200C fan /gas 7. Line a 23 x 33cm Swiss roll tin with baking parchment and oil lightly. Whisk the caster sugar and yolks in a bowl until pale and thick. Melt the chocolate with 4 tbsp cold water in a bowl set over a pan of simmering water. Stir in the sugar mix.
Whisk egg whites to stiff peaks. Stir a spoonful into the chocolate, then fold in remaining using a large metal spoon. Pour batter into tin and bake for 12-14 mins until risen and just firm. Leave in tin until cold.
Lay a sheet of baking parchment on a board. With one bold movement, turn the cake onto the paper, then lift off the tin. Carefully peel away the lining paper.
Spread the whipped cream all over the cake, then scatter over the berries with a dribble of Drambuie, if you like. Starting at the long side opposite you, use the paper to roll the cake towards you. Transfer to a serving dish.
For the buttercream, beat the butter, then sift in sugar and cocoa. Add milk and mix together. Spread over cake, use a fork to create a log effect, and chill until needed. Decorate, dust with icing sugar and serve.