Line the bottom and sides of a 23-24cm (at least 10cm deep) cake tin with baking paper. Heat the oven to 180C/fan 160C/gas 4. Beat the egg whites and cream of tartar until stiff. Put the remaining cake ingredients into a separate bowl with 190ml water and blend to a rich, thick chocolate mixture. Fold in the egg whites. Don’t beat the mixture, as you want to keep the egg whites fluffy. Just fold and fold and fold.
When the mixture has no more visible white streaks, pour it into the tin. Bake for approximately 1 hour. When it is ready it will feel spongy and there will be a soft crust on the top. Cool IN the tin for about 10 minutes then turn it out, peel off the paper and cool on a wire rack.