Preheat the oven to 450 ° F / 220 °C / Gas Mark 8, no fan.
Prepare two (6 oz/170ml) small ramekins (mine have a diameter of 3-inch (7.5 cm), measured at the bottom), grease them generously with butter, then coat with the cocoa powder (important). Dump out any excess cocoa.
In a small pot, melt the butter, when hot (but not boiling), take off the heat. Add broken into small pieces chocolate. Stir until the chocolate is completely melted. Set aside.
In a medium bowl, using a mixer, whisk the egg, sugar, vanilla extract, and salt. Mix for about 1-2 minutes, just until lightly frothy (but it doesn't have to be thoroughly mixed, light and pale).
Add the melted chocolate to the egg-sugar mixture, whisk until just combined.
Add the sifted flour and gently fold to combine.
Divide the batter evenly between the two ramekins (fill it up to approx. ¾ of their height, cakes will rise only slightly). Lightly tap the ramekins, put them in the oven and bake for about 10-11 minutes (10 minutes for a lot of chocolate 'lava', 11 minutes for a medium amount of chocolate 'lava' / very fudgy center).
Remove the ramekins from the oven.