Preheat oven to 375°F. Add 1teaspoon cold water and apinch of salt to the egg whites. Whip egg whites until soft peak stage. Continue to beat and add the sugar, spoon by spoon until egg whites are very stiff. (A spoon should be able to stand up in the meringue if it is beaten stiff enough.)
Gently add the flour, walnuts and another pinch of salt. Fold in, making sure you do not break the egg-white foam.
Line a baking sheet 17 by 12inches with wax paper. Butter the paper lightly and sprinkle with flour; shake off excess.
Spread the batter evenly on the prepared baking sheet. Bake in the preheated oven for 12 to 15 minutes, until firm and golden brown on top.
Cool completely with wax paper over top to keep cake from getting crusty; then cut lengthwise into 3pieces.
Fill the cake layers with chestnut filling and cover sides and top with more of it. Decorate with grated or shaved chocolate. Chill in refrigerator for several hours before serving.