Brush 6 (6 oz) ramekins with soft butter, then coat with sugar and put the ramekins in the freezer.
2. Set a rack in the lower third of the oven and preheat to 400F.
3. Put the chocolate and butter in a bowl and melt over a double boiler until smooth.
4. Remove from heat and stir in vanilla extract.
5. In a separate bowl, combine egg yolks and warm water and beat with a whisk until frothy. A stand mixer may be used as well.
6. Gradually add 2 tablespoons of sugar and continue beating until ribbons form, about 5 minutes.
7. Lightly fold the yolks into the chocolate mixture.
8. Put egg whites in the bowl of a standing mixer, or large bowl, add lemon juice. Beat on medium until frothy, then gradually add the remaining ½ cup sugar and increase speed to high. Beat the egg whites to a stiff peak.
9. Fold about a thirdh of the egg whites into the chocolate to lighten, then flod in the rest until blended.
10. Fill ramekins with mixture and level off the top.
11. Bake until soufflé rises about 1 ½ inches, about 18-20 minutes.
12. Dust with powdered sugar and serve immediately.