To make the base, melt the butter and chocolate over a warm bain-marie. Combine the egg whites and the sugar and beat until stiff. Stir the egg yolks into the melted butter and chocolate. Fold the chocolate mixture and the almonds into the egg white mixture. Spread the combined mixture onto a baking sheet lined with parchment paper. Bake for 25 minutes at 150 °C with the fan on.

For the chestnut cream: soak the gelatine in cold water. Stir the chestnut puree together with the sugar, vanilla sugar, cinnamon, and a dash of rum until smooth. Dissolve the gelatine with a bit of cream over a warm bain-marie. Then lightly whip the rest of the cream. Fold the gelatine into the chestnut puree and then fold in the lightly whipped cream in three steps. (This serves to loosen up the chestnut puree, allowing the cream to retain more of its volume.)

To finish: cut the baked chocolate slab in half. Spread red currant jam on the one half and place the other half on top. Place an appropriately sized metal cake frame around the slab and spread the chestnut cream evenly on top. Freeze and then cut into the desired portions. 

To decorate: prepare a piping bag of chestnut puree mixed with rum and simple syrup. Pipe onto the frozen slices and sprinkle with chopped pistachios.