Prep all the ingredients before you start
Preheat oven to 450°F. Butter and lightly dust four 6oz ramekins with cocoa powder then tap out any excess and set the ramekins on a baking sheet.
Set up your double boiler: Fill a medium saucepan with water to a depth of 1 inch and 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, 5 to 10 minutes. Remove from the heat and set aside.
Beat eggs, egg yolks, sugar, and salt in a mixer fitted with the whisk attachment on high speed until thickened and pale, about 2 to 3 minutes.
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 (depending on your oven).
Remove cakes from oven and let cool in ramekins on baking sheet for 1 minute.
Working quickly 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 with remaining cakes and serve immediately!