1. Preheat the oven to 350°F.
2. Sift the cocoa and flour together. Brush ten 4-ounce ramekins with melted butter and dust with the cocoa-flour mixture.
3. Melt the chocolate and unsalted butter over a double boiler.
4. Whip the eggs with a mixer on high speed until pale yellow and fluffy, about 5 minutes.
5. Fold the chocolate mixture into the eggs.
6. Sift the cake flour and sugar together. Fold the flour-sugar mixture into the chocolate-egg mixture.
7. Fill the ramekins about 3/4 full.
8. Bake for 8 to 10 minutes. The centers should still be runny and the outside edges should be firm. (This can be made ahead of time and stored in the refrigerator up to 2 days.)
9.Unmold each ramekin onto a plate and serve with a pool of raspberry sauce and a dusting of confectioners' sugar.
If reheating, warm in the microwave on low power for two or three 20-second blasts, until warm.
For the raspberry sauce:
10. Puree the berries in a blender with the lemon juice. Blend in the sugar by the tablespoonful, tasting after each addition, until the desired degree of sweetness is reached.
11. Strain through a fine strainer, pressing with a rubber spatula to release the juices.