Butter and dust with about 2 tablespoons of the sugar, 6 (1-cup) souffle dishes. Chill until needed.
Melt the chocolate in a metal bowl set over a double boiler. Remove from the heat and stir in the egg yolks and the liqueur.
Whip all 7 egg whites until they form soft peaks. Whip in the lemon juice and the remaining 2 tablespoons sugar. Continue to whip the whites until they are stiff but still very shiny. Stir a quarter of them into the chocolate mixture, and then gently fold in the remaining whites.
Fill the molds with the souffle mixture. Run your thumb around the inside edge of each dish so that the souffles will form a hat. Bake for 8 to 10 minutes, or until the edges are set but the middle is still just a little soft.
Serve on napkin-lined dessert plates. Dust the tops with powdered sugar, if desired, and spoon a dollop of whipped cream into the center of each souffle. Serve immediately.