Preheat the oven to 375 degrees F (190 degrees C) with oven rack in center position. Butter six 4-ounce ramekins.

Place chocolate and butter into a metal or glass bowl set over a saucepan of barely simmering water. Stir occasionally, scraping down the sides with a rubber spatula to avoid scorching, until melted. Remove from heat.

Combine eggs, sugar, and salt in the bowl of a stand mixer fitted with the paddle attachment; beat at medium-high speed until light and fluffy, about 5 minutes. Reduce speed to low; add chocolate mixture and beat until combined. Divide batter among the prepared ramekins; place on a baking sheet.

Bake until edges are set but center is underdone, about 10 minutes. Cool before serving for 3 minutes.