Preheat your oven to 180°C (160°C fan)/ 350°F.
Melt the butter in a small pot.
Put the sugar, flour and bicarbonate of soda into a medium mixing bowl. Pour the butter in and stir well to get a crumbly mixture. Stir in the egg yolk and vanilla to get a loose cookie dough. Fold in the chocolate chip (reserving a few for the top).
Divide the cookie dough between two 4oz oven-proof ramekins (see notes if you don't have them). Flatten out the cookie dough and sprinkle on the reserved chocolate chips and some flaky salt if you want.
Place onto a baking tray to make it easier to get them out of the oven. Bake for 9-12 minutes - the edges of the cookie dough should be set, the top should look dry and the centre should still be soft when gently poked.
Remove from the oven, let cool for 5 minutes, then serve.