Line base and long sides of two 8 by 26cm bar cake pans with baking paper. Combine currants and brandy in a bowl. Cover and set aside for 1 hour.
Place butter and chocolate in a heatproof bowl. Stir over simmering water until melted and smooth. Using an electric beater, beat eggs and sugar together until pale and thick. Stir in chocolate mixture, currants and brandy. Gently fold in remaining ingredients until just combined.
Spoon into prepared pans and spread out. Cover surface with plastic wrap and smooth out. Refrigerate overnight, until set. Remove plastic wrap, turn out onto a board and dust with cocoa. Slice and serve, or cut in half lengthways and wrap to give as a gift.