Preheat oven to 450°F. Butter and lightly flour four 6-ounce ramekins. Tap out excess flour. Set the ramekins on a baking sheet.

Fill a medium saucepan with water to a depth of 1 inch; bring to a boil over medium-high and reduce heat to medium-low to maintain a simmer. Combine chocolate and butter in a medium-size heatproof bowl and set bowl over simmering water in pan, making sure bottom of bowl doesn’t touch water. Cook over medium-low, stirring occasionally and adjusting heat as needed to maintain a simmer, until chocolate mixture is melted and smooth, 16 to 20 minutes. Remove from heat and set aside.

Beat eggs, egg yolks, sugar, and salt in a medium bowl with an electric mixer fitted with the whisk attachment on high speed until thickened and pale, 2 to 3 minutes

Whisk melted chocolate until smooth.

Quickly fold melted chocolate mixture and flour into egg mixture in bowl until well combined and no streaks remain.

Spoon batter into prepared ramekins (about 1/2 cup per ramekin) and smooth the top using a small offset spatula if needed. Place baking sheet with ramekins in the preheated oven; bake until tops of cakes are just set and edges spring back when touched but centers still feel soft, about 9 to 12 minutes.

Remove cakes from oven and let cool in ramekins on baking sheet for 1 minute. Working with one ramekin at a time, cover ramekin with an inverted dessert plate. Carefully turn over and let stand 10 seconds before lifting the ramekin to unmold cake. Repeat unmolding procedure with remaining cakes. Serve immediately.