Set your oven to 425°F (220°C). Grease a 6-ounce ramekin and dust with flour, shaking out the excess.
In a microwave-safe bowl, melt the chocolate and butter in 20-second intervals, stirring between each, until smooth. Make sure your chocolate has cooled a bit before adding it to your eggs (see step 4)
In a separate bowl, whisk the egg, egg yolk, and sugar until the mixture is pale and slightly thickened. Stir in the vanilla.
Gently fold the melted chocolate mixture into the egg mixture.
Sift in the flour and salt, and fold until just combined.
Pour the batter into the prepared ramekin.
Place the ramekin on a baking sheet and bake for 10-12 minutes. The edges should be set, but the center should still jiggle slightly.
Let the cake cool for about 5 minutes, then run a knife around the edges and invert onto a serving plate. Dust with powdered sugar and serve warm with ice cream or berries.