Beat the egg whites with the salt until stiff. Add half the sugar and continue beating until the egg whites turn glossy. Mix the remaining sugar with the hazelnuts and flour and carefully fold into the egg whites using a rubber spatula. Spoon the mixture into a piping bag with a smooth nozzle (approx. 1 cm diameter), pipe around 60 fingers (each approx. 4 cm long) onto two baking trays lined with baking paper.
Bake for approx. 15 mins. in the centre of an oven preheated to 160 °C (convection). Remove, immediately separate from the oven tray with a spatula, allow to cool on a rack.
Bring the cream to the boil, remove the pan from the heat, add the chocolate and vanilla seeds and melt, stirring. Leave to stand for approx. 15 mins.
To fill, lay out half the fingers with the flat side up. Divide the slightly thickened filling over the fingers. Top with the rest of the fingers, flat side down. Leave to set in the fridge, on a rack, for approx. 1 hr. Dust with icing sugar.