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.
This is my favorite chocolate recipe to make ahead and keep in the freezer until I am ready to heat and eat. I freeze them in paper cupcake liners in a cupcake/muffin pan and then transfer them to a freezer container for storage.They last months in a good freezer container because it takes me that long to eat this recipe. I usually bake just one at a time in my toaster oven.