1. Lightly butter four ramekins (2/3-cup size, deep or shallow). Set ramekins in a shallow, 10- by 15-inch baking pan.
2. Combine chocolate and milk in a microwave-safe bowl; heat at full power (100%) in a microwave oven at 30-second intervals, stirring after each, until chocolate is melted and smooth, 30 seconds to 1 minute total. Let stand.
3. In a large bowl, with a mixer at high speed, whip egg whites until they form a dense foam, 1 to 2 minutes. Sprinkle in 1/4 cup of the sugar and beat until whites hold short, distinct peaks, 1 to 4 minutes. Scrape into another bowl.
4. In mixer bowl, whip egg yolks at high speed until they begin to thicken, 1 1/2 to 2 minutes. Add remaining 2 tablespoons sugar and continue beating until yolks are very thick, 3 to 5 minutes. Beat in vanilla. Add melted chocolate and beat until well blended.
5. Add about a third of the egg whites to egg yolk and chocolate mixture, and beat slowly to mix well. Fold in remaining whites until fairly well incorporated.
6. Scoop soufflé mixture equally into ramekins; mixture can mound above rims, but make sure it's not dripping over edges.
7. Bake in a 375° oven until soufflés are puffy and dry on the surface, about 8 minutes for very soft centers, 10 minutes for creamy centers, and 12 minutes for firm centers.
8. Transfer ramekins to plates and rub powdered sugar through a fine sieve onto soufflés. Offer whipped cream to top soufflés.