Preheat oven to 170 degrees C. Line and grease an 18 cm pan. Melt together 200 g butter and 200 g baking chocolate in a bowl set over a saucepan of simmering water. Remove from heat. Cool a bit before adding the rest of the ingredients.
Add the sugar gradually. Stir the mixture as you add the sugar. Add egg one at a time, stir after each addition. Lastly, add the flour.Mix. Pour into the pan. Bake for half an hour. Do not open the oven! Doing so will shrink the cake. Leave the cake in the oven for another 10 minutes to rest. Cool completely before inverting onto a plate. Serve.