Combine the butter, sugar and egg yolks in a bowl and mix well. Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie; the bowl must not touch the water. Melt the chocolate, stir until smooth, then stir into the mixture. Beat the egg whites with the salt until stiff. Mix the almonds and flour, add to the mixture in layers along with the egg whites, carefully fold in using a rubber spatula. Transfer to the prepared tin.
Bake for approx. 20 mins. in the lower half of an oven preheated to 180°C. Allow to cool a little in the tin.
Bring the redcurrants to the boil and reduce the heat. Add the lemon juice and gelling agent, continue to simmer for approx. 1 min., stirring constantly. Stir in the sugar and simmer for a further 1 min. Spoon the redcurrant mixture over the cake and allow to cool in the tin. Cover and chill for approx. 30 mins. Using the whisk attachment on a hand mixer, whisk the cream, crème fraîche and icing sugar until light and fluffy.
Remove the cake from the tin and cut into 10 rectangles. Shortly before serving, spoon the whipped cream on to the redcurrant slices and dust with cocoa powder.