Put chocolate and butter in a medium metal bowl and set bowl over (not touching) simmering water in a pot. Heat until chocolate is almost completely melted; remove from heat, stir until smooth, and set aside.

With a mixer fitted with the whisk attachment, whisk egg whites and salt in a bowl until foamy. With mixer still on, slowly rain in sugar and whisk until soft peaks form.

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

Preheat oven to 325°. Set ramekins on a rimmed baking sheet, remove plastic wrap, and bake until puffed, cracked on the edges, and just wet-looking in the center, 30 to 35 minutes. Let cakes cool 10 minutes before serving.