Preheat your oven to 425°F (220°C). Generously butter four 6-oz ramekins and dust the insides with cocoa powder, tapping out any excess. Place on a baking sheet and set aside.
In a heatproof bowl set over a pot of simmering water (or in the microwave in 30-second intervals), melt the chocolate and butter together, stirring until completely smooth. Remove from heat and let cool for 3 to 4 minutes.
In a medium bowl, whisk together the eggs, egg yolks, sugar, salt, and vanilla until the mixture is slightly pale and combined, about 1 to 2 minutes. Slowly pour the warm chocolate mixture into the egg mixture, whisking constantly. Fold in the almond flour or GF baking flour until just incorporated.
Divide the batter evenly among the prepared ramekins. At this point, you can bake immediately or cover and refrigerate for up to 24 hours. Bake for 11 to 13 minutes, until the edges are set and the tops look mostly matte with a faint jiggle in the very center.
Let the cakes rest in the ramekins for 1 minute. Run a thin knife around the edge, place a small plate on top, and carefully invert. Serve immediately with a dusting of powdered sugar or a scoop of vanilla ice cream.