If baking immediately, preheat oven to 375° F.
Combine chocolate and butter in a medium heatproof bowl. Set the bowl over a saucepan of barely simmering water, stirring frequently until melted and smooth. Remove the bowl from the saucepan to cool slightly.
In the bowl of a standing electric mixer fitted with a whisk, mix eggs to combine. With the mixer on low speed, pour in the sugar and add the salt. Increase speed to medium-high and whisk until the eggs are pale yellow and fluffy, about 5 minutes.
Fold about 1/3 of the egg mixture into the warm chocolate mixture. Scrape the chocolate mixture into the remaining eggs and fold gently until evenly combined.
Divide the batter between 6 ungreased ramekins.
To bake right away, set the ramekins on a baking sheet. Place them in the lower rack of the oven and bake until the tops puff up, turn crisp, and crack, revealing gooey centers below, about 15 to 20 minutes.
To prepare ahead, cover the ramekins and refrigerate them for up to 1 day. Let them sit at room temperature for about 30 minutes while the oven preheats to 375° F. They'll take longer to bake after refrigeration, about 20 to 25 minutes.
Serve warm or at room temperature with toppings of your choice.