Grease two oven-safe ramekins with soft butter using a pastry brush, covering both the base and sides thoroughly. This step is crucial.
Preheat the oven to 375°F (190°C).
In a bowl, melt the dark chocolate and butter together in the microwave in 30-second bursts, or melt them over a double boiler, until smooth.
Add the milk and vanilla extract to the melted chocolate-butter mixture and stir until fully combined.
Sift in the powdered sugar, all-purpose flour, cornstarch, baking powder, cocoa powder, and salt. Mix gently until just combined.
Pour the batter evenly (130g each) into the greased ramekins.
Bake for 12-15 minutes, or until the sides are firm and the center is slightly risen.
Remove from the oven and let the cakes cool for 5 minutes.
Gently invert the cakes onto serving plates.
Dust with powdered sugar and serve with a scoop of ice cream.