An aura of mystery surrounds this showstopping dessert, which is probably why so few cooks try to make it at home. In fact, it is easy to make if you pay close attention to the baking time. Find this recipe and many more delicious treats in The Old Farmer’s Almanac Everyday Baking cookbook.
Combine all of the butter and the chocolate in the top of a double boiler over not-quite-simmering water. When the chocolate has melted, whisk to smooth. Set aside to cool for 15 minutes. Stir in the vanilla. Meanwhile, preheat the oven to 450°F. Butter six 1-cup ceramic ramekins. (Do not substitute tall custard cups.) Put 1 teaspoon of sugar in each. Turn and tilt the ramekins to coat them evenly with sugar. Set aside. Whisk the eggs and yolk in a large bowl, until blended. Add the confectioners’ sugar and whisk to combine. The mixture will look curdled. Add the chocolate to the egg mixture and whisk until evenly blended. Add the flour and salt, then whisk to blend. Divide the batter evenly among the ramekins (they’ll be about half full).
Bake one as a trial to check the timing: Place it directly in the middle of the center oven rack. Immediately set your timer for 10 minutes and bake, expecting it to take 30 to 60 seconds either way.
When done, the cake will have doubled in height; the top will be almost flat, not sunken or knotted; and a “skin” will have formed around a soft, flowing center. Using a spatula and oven mitts, immediately remove the dish from the oven. Place a dessert plate over the ramekin and invert the cake onto the plate. Wait about 10 seconds, then carefully lift the ramekin. If this is done properly, you will have a nicely shaped cake that will be slightly sunken in the middle, with a soft, lavalike, but not overly runny center. (Use a paring knife to cut into the top to see; cover the cuts later with a scoop of ice cream.)
Bake the remaining cakes, adjusting the time as needed, and invert them as directed. If time requires, set the cakes aside for several hours before serving or cover with plastic wrap and refrigerate on the plates. When you’re ready to serve, remove the plastic and reheat in the microwave to soften the centers: 5 to 10 seconds for a room temperature cake; 10 to 15 seconds for a cold cake. Serve with a scoop of vanilla ice cream on top.