Set your oven to 200°C (400°F). Grease 2 small ovenproof dishes or ramekins with butter and place them on a baking tray.
Chop the chocolate if necessary and place in a microwaveable dish. Add the butter and microwave in 30-second bursts until melted. In my microwave, it takes 2 minutes on medium power. Remove, stir gently and allow to cool a little (to avoid making chocolate scrambled eggs).
In a medium bowl, beat the eggs and sugar together with the salt and vanilla until very well combined. You could use a handheld mixer, though I prefer to do it by hand with a balloon whisk because it's quieter and more pleasant.
Stir in the slightly cooled chocolate, then mix in the flour until just combined. Do not over-beat.
Divide the mixture between the dishes and cook for 12 minutes. Check that the top is just beginning to crack and remove from the oven. Let them sit for 5 minutes before serving, turning them out onto a plate if you want to.
While these are perfectly delicious just the way they are, you could serve them with cream or vanilla ice cream to suit your mood.