Set the middle oven rack in place and heat the oven to 375°F (190°C). This positions the molds in the center of the oven for baking.
Cover the bottoms and walls of four 6-oz (180-ml) soufflé molds generously with softened butter, then rotate sugar through each mold, passing the excess to the next and discarding what remains. Look for an even sugar coating over the buttered surfaces, without bare patches.
Use a medium saucepan over medium heat to melt the batter butter, then whisk in the flour for about 1 minute. The butter-flour mixture should bubble and look smooth.
Whisk in the cocoa, followed by the milk, optional liqueur or replacement milk, vanilla, and salt, then stir continuously for about 2 minutes. Stop cooking when the mixture thickens into a smooth paste with gentle bubbles.
Take the saucepan off the burner, whisk in the chopped chocolate until melted, then promptly incorporate the yolks individually and set the mixture aside. The finished chocolate mixture should look uniform and smooth, without unmelted pieces.
Use an electric mixer’s whisk attachment at medium-high speed to beat the whites for about 1 minute, then gradually add the sugar and beat for about 2½ minutes more. The whites should progress from soft peaks to shaped, supple peaks with tips that bend slightly, rather than becoming dry or rigid.
Gently whisk one-third of the whipped whites into the chocolate, then transfer that loosened mixture into the remaining whites and incorporate it with a rubber spatula using gentle folds. Finish when the batter has an even chocolate color without visible streaks of white.
Distribute the batter evenly among the molds to about ⅛ inch below their tops, wipe the rims clean with your fingers, and arrange the molds on a baking sheet. The upper edges should be free of batter drips that could catch during baking.
Bake for about 15 minutes, checking for doneness with a light shake of the baking sheet. The tops should rise above the rims, the outsides should look set, and the centers should wobble gently.
Support each hot ramekin with two spatulas to move it onto a serving plate, then add confectioners’ sugar if desired and serve immediately. The soufflés should reach the table while their tops remain puffed.