Brush the inside of six 5-ounce ovenproof ramekins or Pyrex custard cups with the softened butter. Put the cocoa powder in a ramekin and rotate the ramekin so the cocoa completely coats the inside walls. Pour the cocoa powder from one ramekin to another until they are all coated. Tap firmly on the bottom of each ramekin after coating it with cocoa so that there are not clumps of cocoa left clinging on the inside. Put the ramekins on a sheet pan and slide them into the refrigerator.

Combine the 6 tablespoons of unsalted butter, the chocolate, and the vanilla extract in a small heatproof mixing bowl. Place the bowl over a saucepan of simmering water (but don’t let the bowl touch the water) and stir with a whisk until smooth. Or, using a nonmetal bowl, heat in the microwave for 30 seconds.

Combine the eggs and sugar in the bowl of an electric mixer. They take a lot of beating, so try to avoid beating them by hand. Whip on high speed for 2 to 4 minutes with the whisk attachment until the eggs triple in volume – eggs at room temperature froth up more quickly than cold ones. When you lift the whisk about 6 inches out of the bowl, the mixture should slowly fall in a steady stream – bakers call this a ribbon – about 1 inch thick.

Scoop the egg mixture over the chocolate mixture. Put the flour in a strainer and sift a small amount over the egg-chocolate mixture by gently shaking the strainer. Fold everything together with a rubber spatula, sifting the flour over the mixture about four times during the folding. Pour the mixture into the ramekins, filling them about three-fourths full. Put the souffles in the freezer for at least 1 hour but no longer than 1 week.

Thirty minutes before serving, preheat the oven to 425F. Slide the sheet pan with the ramekins into the oven and bake for 17 minutes, until the mixture has risen about 1/4 inch above the edges. The souffles should be cakelike on the outside and still runny in the middle.

You can serve the souffles in their mold – just sprinkle them with a bit of confectioners’ sugar – or you can quickly run a knife around the inside of the ramekins and unmold the cakes on hot plates. Serve with sweetened whipped cream or vanilla ice cream.