Make the Chocolate Mixture: In a heatproof bowl set over a saucepan of simmering water (double boiler), combine chocolate and butter. Stir occasionally until melted and smooth. Remove from heat and let cool slightly.  

Whisk in Egg Yolks and Sugar: In a separate bowl, whisk together egg yolks and 1/4 cup granulated sugar until light and fluffy. Gradually whisk in the cooled chocolate mixture until well combined.

Fold in Flour and Salt: Sift flour and salt together. Gently fold the flour mixture into the chocolate mixture until just combined. Be careful not to overmix.

Whip Egg Whites: In a clean bowl, beat egg whites with an electric mixer on medium speed until foamy. Gradually add 1 tablespoon of sugar and continue beating on high speed until stiff peaks form.

Fold in Egg Whites: Gently fold about 1/3 of the egg whites into the chocolate mixture to lighten it. Then, fold in the remaining egg whites in two additions until just combined.

Prepare Ramekins: Butter six 6-ounce ramekins or ovenproof cups. Divide the batter evenly among the ramekins.

Bake: Preheat oven to 400°F (200°C). Bake the cakes for 10-12 minutes, or until the edges are set but the centers are still slightly jiggly.

Serve Immediately: Carefully invert the ramekins onto serving plates. The warm chocolate center will ooze out. Dust with powdered sugar and serve immediately with vanilla ice cream, if desired.