Steps
Preheat your oven to 425°F (220°C).
In a microwave-safe bowl, melt the dark chocolate and butter together, stirring until smooth.
In another bowl, whisk the egg and sugar until light and fluffy.
Combine the melted chocolate mixture with the egg mixture.
Stir in the flour, salt, and vanilla extract until just combined.
Grease a ramekin with butter and pour the batter into it.
Bake for 12-14 minutes until the edges are firm but the center is soft.
Let it cool for a minute, then invert onto a plate and serve immediately.