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.
Lightly beat yolks with a fork, and then stir 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) or 16 shallow heart-shaped ramekins (3 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 (or 15 to 20 minutes for 3-oz. ramekins). Let cakes cool 10 minutes before serving. Top with whipped cream and dust with cocoa, if you like.
Makes 12 4-oz. ramekins or 16 3-oz. heart-shaped ramekins