For 12 people:
240g castor sugar
100g of potato starch/flour
1/2 tsp baking powder (and incidentally, the French don't measure in 'teaspoons' but in coffeespoons. One of those intriguing cultural differences.)
Melt the chocolate in a bain-marie with 2 soupspoons of water; add the butter and gently pour the wet mixture over the creamed sugar and eggs.
Little by little add the potato starch, baking powder and egg whites whipped into stiff peaks.
Bake in a medium hot oven (200C) in a 23/24cm greased and floured tin. Don't open the oven door in the first 20 minutes; takes about 30/35 minutes to bake.
The result is a moist, decadent cake. You can substitute regular flour but the potato starch not only makes the cake extra moist, it also makes it suitable for coeliacs or anyone who can't eat wheat.
I will now channel Julia Child and say - bon appetit!