Steps
Preheat your oven to 425°F (220°C).
Grease a ramekin with butter and dust with cocoa powder.
In a microwave-safe bowl, melt the butter and dark chocolate together until smooth, stirring occasionally.
Remove from the microwave and whisk in the powdered sugar until well combined.
Add the egg, egg yolk, vanilla extract, and a pinch of salt; whisk until fully incorporated.
Gently fold in the flour until just combined, being careful not to overmix.
Pour the batter into the prepared ramekin.
Bake for 12-14 minutes until the edges are firm but the center is still soft.
Let it cool for a minute, then invert onto a plate and serve immediately to enjoy the molten center.