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 dark chocolate and butter together in 30-second intervals, stirring until smooth.
In a separate bowl, whisk together the egg, egg yolk, and sugar until light and fluffy.
Gently fold the melted chocolate mixture into the egg mixture.
Sift in the flour and salt, folding until just combined.
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 1 minute, then invert onto a plate.
Serve immediately with a scoop of vanilla ice cream on the side.