1. Preheat the oven to 450°F (220°C).
2. Cut the butter and chocolate (gluten-free, make sure there is no gluten) into pieces. Put them in a saucepan and melt them over low heat while stirring.
3. Off the heat, add the sugar and the egg yolks one by one, stirring each time.
4. Gently fold in the beaten egg whites.
5. Finally, add the cornstarch and mix again.
6. Pour the batter into a greased 8-9 inch (22-24 cm) diameter cake pan.
7. Bake in the oven at 450°F (220°C) for 12 to 15 minutes. The inside of the cake should remain moist.
8. Let the cake cool before unmolding, then refrigerate.
9. Serve with custard sauce or a red fruit coulis.