1. Preheat the oven to 350⁰F.
2. Chop the chocolate, leaving 6 squares intact.
3. Melt the butter and 150g of chocolate in a double boiler. Mix with a wooden spoon.
4. In a bowl, beat the eggs and sugar until the mixture is pale. Add the melted chocolate and mix gently. Add the flour, baking powder, and fleur de sel; mix.
5. Grease the individual molds. Fill halfway with dough and place 1 reserved chocolate square in the center.
6. Divide the remaining dough among the molds and bake for 10 minutes.
7. Remove from the oven. Unmold gently and serve immediately.