2) Place the egg whites, salt and cream of tartar in the bowl of an electric mixer fitted with the whisk attachment and beat on high speed until the eggs form medium-firm peaks, about 1 minute. With the mixer on medium speed, add the remaining 300g of sugar by sprinkling it over the beaten egg whites. Beat on high speed for a few minutes until thick and shiny.
3) Add the vanilla and continue to whisk until very thick, about 1 more minute. Scrape the beaten egg whites into a large bowl. Sift 1/4 of the flour mixture over the egg whites and fold it very carefully into the batter with a rubber spatula. Continue adding the flour in 3 equal additions, sifting and folding until it's all incorporated. Fold in the grated chocolate.
4) Pour the batter into an ungreased 25cm tube tin, smooth the top, and bake it for 35 to 45 mins, until it springs back to the touch. Remove the cake from the oven and invert the tin on a cooling rack. When cool, run a thin, flexible knife around the cake to remove it from the tin.
5) For the chocolate glaze, place the chocolate chips and the double cream in a heat-proof bowl over a pan of simmering water and stir until the chocolate melts. Pour the chocolate over the top of the cooled cake to cover the top completely and allow it to drizzle down the sides. If you have chocolate glaze left over, you can serve it on the side with the cake.