Cut the chocolate into pieces, melt over a bain-marie.
Beat together the sugar and egg yolks using the whisk on a hand mixer until the mixture becomes lighter in colour. Stir the chocolate into the egg yolk mixture.
Beat the egg whites with the salt until stiff. Add the sugar and continue beating until the egg whites turn glossy. Add the flour to the mixture in layers along with the egg whites, fold in carefully.
Spread the sponge mixture into a rectangle (approx. 7 mm thick) on the back of a baking tray lined with baking paper.
Bake for approx. 15 mins. in the centre of an oven preheated to 180°C. Remove from the oven.
Tip the sponge and paper onto a fresh sheet of baking paper, carefully peel away the paper. Cover the sponge immediately with the upturned tray, leave to cool.
Mix the mascarpone, sugar and pistachios in a bowl. Grate in the lemon zest. Add 2 tbsp of the lemon juice, mix.
Cut six raspberries in half for the decoration and chill. Roughly mash the remaining raspberries and mix into the filling.
Beat the cream until stiff, cover and refrigerate 1/3 of the cream, carefully fold the remaining cream into the filling.
Trim the edges of the sponge and cut in half crosswise. Spread the filling over one half, place the other half of the sponge on top, press down gently, cover and freeze for approx. 1 1/2 hrs.
Cut into 12 rectangles. Decorate with the reserved cream, a few chocolate shavings and the reserved raspberries.