Molten Chocolate Lava Cakes

Generously butter the inside of four 6-ounce ramekins, then dust each with cocoa powder, tapping out the excess -- this coating is what lets the cakes release cleanly later.
Preheat the oven to 425F and place a baking sheet inside to preheat as well.
In a microwave-safe bowl, combine the chopped chocolate and butter. Microwave in 30-second bursts, stirring after each, until fully melted and smooth, about 90 seconds total.
In a separate bowl, whisk the eggs, egg yolks, granulated sugar, and salt together vigorously for about 1 minute, until slightly thickened and pale.
Whisk the melted chocolate mixture into the egg mixture until fully combined.
Gently fold in the flour and vanilla with a spatula just until no dry streaks remain -- overmixing at this stage can make the cakes tough instead of tender.
Divide the batter evenly among the prepared ramekins, filling each about 3/4 full.
Place the ramekins on the preheated baking sheet and bake for 11-13 minutes, until the tops are set, puffed, and slightly cracked, but the very center still looks soft when gently pressed -- underbaking by even 1 minute is the difference between molten and fully cooked through, so watch closely near the end.
Remove from the oven and let the cakes rest in the ramekins for exactly 1 minute -- this brief rest helps the outer cake firm up enough to unmold without collapsing.
Run a thin knife around the edge of each ramekin, place a small plate upside down on top, and carefully invert to release the cake. Lift the ramekin away.
Dust with powdered sugar and serve immediately while the centers are still warm and flowing.