Brush 6 (6-ounce) ramekins with soft butter, then coat with sugar. Put the prepared ramekins in the freezer. (This can be done a day ahead.)
Set an oven rack in lower third of the oven and preheat to 400 degrees F.
Put the chocolate and butter in a medium heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl over, but not touching, the water. Stir the chocolate occasionally until melted and smooth. Remove from heat and stir in vanilla extract. Set aside.
Combine the egg yolks and warm water in the bowl of a standing mixer or large bowl and beat until frothy. Gradually add 2 tablespoons sugar, and continue beating until ribbons form, about 5 minutes. Very lightly fold the yolks into the chocolate mixture. (Rinse the bowl well, if using for beating the egg whites.)
Remove prepared ramekins from freezer. Put the egg whites in the bowl of a standing mixer, or large non-reactive bowl, add the lemon juice. Beat on medium until frothy; then gradually add the remaining 1/2 cup of sugar and increase speed to high. Beat until the whites hold a stiff but not dry peak.
Working quickly, fold about a third of the egg whites into the chocolate to lighten; then fold in remaining whites until blended. Gently ladle or spoon the souffle mixture into the ramekins, and place on a baking sheet. (Level off the surface with a straight edge, scraping any excess mixture back into the bowl.)
Immediately bake until the souffle rises about 1 1/2 inches from the ramekins, and the tops are touched with brown, about 18 to 20 minutes. Remove from oven, dust with confectioners' sugar and serve immediately.
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