1 Preheat oven to 180°C (160°C fan) mark 4. Lightly grease and line a Swiss roll tin (about 23x33cm, 9x14in) with baking parchment. Sprinkle extra caster sugar on another piece of baking parchment twice the size of the tin. Set aside.
2 In a large heatproof bowl set over a pan of simmering water, beat caster sugar and eggs with a handheld electric whisk for 6-7min, until thick, pale and moussey (the beaters should leave a ribbon-like trail on the surface when lifted out of the mixture). Remove bowl from heat, add vanilla and whisk for 5min, until cool.
3 Sift flour and a pinch of salt over egg mixture and fold in with a large metal spoon, trying to keep in as much volume as possible. Carefully pour mixture into prepared tin and spread to level with a palette knife. Bake in middle of the oven for 15min, or until springy to the touch.
4 As soon as the sponge is baked, invert on to the sugared parchment. Remove tin. Peel off and discard lining parchment. While warm, gently roll sponge up from a short side, using parchment underneath to help. Set aside (wrapped in its parchment) on a wire rack to cool.
5 Meanwhile, make the blackcurrant compote. Heat blackcurrants, sugar and 2tbsp water in a small pan over low heat until berries are bursting. Cool.
6 For the filling, using a handheld electric whisk, beat cream, condensed milk and vanilla until mixture holds soft peaks. Add cooled chocolate and beat until it forms firm peaks. Chill for 30min to firm up.
7 Unroll cooled sponge on to a large piece of clingfilm and discard parchment. Spread filling over sponge, leaving a 1cm (½in) border around edges. Drizzle over blackcurrant compote. Roll sponge up again gently from the short edge, then wrap tightly in the clingfilm. Place on a baking tray seam-side down and freeze for 4hr, or until filling is firm.
8 For the drizzle, heat blackcurrants and 1tbsp water in a small pan until gently bubbling. Remove from heat, add white chocolate and stir to melt. Pass through a sieve. Remove roulade from freezer, unwrap and place on a serving plate or board. Dust with icing sugar. Spoon or pipe over the drizzle and serve.
