Preheat your oven to 425°F (220°C). Lightly grease two 6-ounce ramekins with butter, then dust the insides with a little cocoa powder or flour.
In a heatproof bowl, combine the chocolate and butter. Melt them together either using a double boiler or the microwave in 20-second intervals, stirring each time until smooth and glossy.
Once melted, set aside to cool slightly. You don’t want it too hot when you mix in the eggs — warm is perfect.
In a separate bowl, whisk together the egg, egg yolk, sugar, and vanilla extract until light and slightly frothy. You can use a hand whisk or an electric mixer.
Pour the melted chocolate mixture into the egg mixture and whisk gently to combine. Then add the flour and a pinch of salt, folding just until the batter is smooth and shiny.
Divide the batter evenly between the two prepared ramekins. They should be about three-quarters full.
Bake for 12–14 minutes, depending on your oven. The edges should look set and slightly firm, but the centers should still be soft and jiggly when you gently shake the ramekins.
Remove the ramekins from the oven and let them sit for 1 minute. Then, run a small knife around the edges to loosen the cakes.
Place a plate on top of each ramekin, flip it over, and gently lift off the ramekin. The warm cakes should slide out perfectly — slightly firm on the outside with molten chocolate waiting inside.
Dust with powdered sugar or cocoa powder, and if you’re feeling fancy, add a scoop of vanilla ice cream on top.
