5 tablespoons unsalted butter, cut into small cubes, plus more for greasing
1 1/2 teaspoons pure vanilla extract
8 ounces bittersweet chocolate, chopped
Pinch of sea salt
1/4 cup unbleached all-purpose flour
2 large eggs Whipped cream, crème fraîche, or vanilla ice cream for serving
Position a rack on the lowest level in the oven and preheat to 400F (200C). Lightly butter six 1/2-cup (4 fl–oz/125-g) ramekins.
Put the butter and chocolate in a heatproof bowl and place over (not touching) simmering water in a saucepan. Heat until melted, then stir until smooth. Remove from the heat and let cool slightly.
Meanwhile, in a bowl, combine the sugar, eggs, vanilla, and salt. Using an electric mixer set on high speed, beat until thickened, about 3 minutes. Reduce the speed to low.
Gradually sprinkle the flour over the batter and continue beating just until combined. Using a spatula, fold the egg mixture into the cooled chocolate mixture until combined.
Divide the batter among the prepared ramekins. Place the ramekins on a rimmed baking sheet and bake until the tops are puffed and dry and a wooden skewer inserted in the center comes out with some soft batter clinging to it, about 10 minutes.
Serve the cakes hot, in the ramekins, or inverted onto individual plates (right). Top with a dollop of whipped cream, if you like.
Dress it up: Line the bottom of each ramekin with a parchment-paper round, then pour in the batter. (This lining step is painstaking, but the cakes will unmold more easily.)
Bake as directed, then remove from the oven. Immediately grasp a ramekin with a pot holder or thick towel, run a thin-bladed knife around the inside edge to loosen the cake, and invert the cake onto a small plate or into a small bowl.
Peel off the parchment and serve.