Preheat oven to 165 degrees C. If it does not have a removable bottom, line a 22cm ring tin with baking paper.
Sift together the flour, cocoa andthe sugar. Set aside. Measure 3 tablespoons of the flour mixture and toss with the grated chocolate.
In a large bowl, using an electric mixer set at low speed, beat egg whites until foamy. Increase mixer speed to medium then stir in cream of tartar and salt. Continue to beat until egg whites form soft peaks. Gradually mix in the remaining 3/4 cup sugar and beat until stiff peaks form. Stir in the vanilla and lemon juice.
Gently fold the flour mixture into the beaten egg whites, then fold in the grated chocolate, stirring just until no white streaks remain.
Gently scrape the batter into the prepared cake tin. Smooth the surface of the batter and tap the tin lightly to remove any air bubbles. Bake in centre of the preheated oven for 60 minutes, or until the surface springs back when touched. Cool completely and sprinkle with icing sugar before serving.