1 hr 10
By Frédéric Bau and École du Grand Chocolat Valrhona
A day ahead. Rinse the sultanas under boiling water and plump them up by simmering over low heat in the white rum and water until the liquid has been completely absorbed.
Prepare the cake batter. Place the butter in a saucepan over low heat until just melted. Chop the chocolate and melt it slowly in a bain-marie