Prep your ramekins: Grease four 6-ounce ramekins with butter, then coat the insides with granulated sugar. Tap out any excess. Place them on a baking sheet.
Melt the chocolate and butter: In a heatproof bowl over simmering water (or use a microwave in short bursts), melt the chocolate and butter until smooth. Let it cool slightly.
Mix yolks and flavoring: Whisk in the egg yolks, vanilla, and a pinch of salt into the melted chocolate until glossy.
Whip egg whites: In a clean mixing bowl, beat the egg whites with cream of tartar until soft peaks form. Gradually add sugar and beat until stiff, glossy peaks form. (Yes, it should hold a peak like a little chocolatey mountain!)
Fold it all together: Gently fold a third of the egg whites into the chocolate mixture to lighten it. Then carefully fold in the rest until just combined. Don’t overmix!
Fill and bake: Spoon the batter into prepared ramekins, smoothing the tops. Run your thumb around the inside edge of each ramekin (this helps them rise evenly!).
Bake: Pop them into the oven and bake for 12-14 minutes, until puffed and set but still a bit wobbly in the center.
Serve immediately: Dust with powdered sugar and serve right away—these beauties wait for no one!