1. In a medium pot, bring 1 in. water to a simmer. Put chocolate and butter in a medium metal bowl and set bowl over pot, being careful not to let bottom of bowl touch water. Heat until chocolate is almost completely melted; remove from heat, stir until smooth, and set aside.

2. With a mixer on medium speed, whisk egg whites and salt in a bowl until foamy. With mixer still on, slowly rain in sugar and whisk until soft peaks form.

3. Stir yolks into chocolate mixture until completely blended. Using a rubber spatula, gently fold in whites. Spoon mixture into 12 small freezer- and ovenproof coffee cups (4 oz. each). Cover each cup with plastic wrap and freeze until ready to serve, at least 2 hours and up to 1 week.

4. Preheat oven to 325°. Set cakes on a baking pan and bake until puffed, cracked on the edges, and wet-looking in the center, about 30 minutes. Let cool 10 minutes before serving.

Note: Nutritional analysis is per serving.