Melt the chocolate, butter and a pinch of salt together in a big mixing bowl over a pan of barely simmering water. Stir in the ground almonds, followed by the raisins and any remaining rum in the bowl and take off the heat. Beat the egg whites until stiff, add the caster sugar and beat again until stiff-ish and shiny.
Stir 1⁄4 of the whites into the chocolate mixture to loosen, then gently fold in the rest with a big metal spoon or spatula. Spoon into the tin, then bake for 30-35 minutes until the centre still has a slight wobble. Cool in the tin, then turn upside down onto a serving plate, leave the paper on, cover and chill overnight.