Preheat the oven to 170 C. Lightly grease two ramekins (3.5 inches in diameter and 2 inches in height). See notes for alternatives.
Place the butter in a microwave-safe mixing bowl and melt it, making sure it's not boiling hot. Stir to melt down any remaining lumps, then whisk in both the sugars, mixing until dissolved.
Add the egg yolk and vanilla and stir well to incorporate. Sift in the flour, baking soda and salt, if using. Gently fold till just combined. Finally, stir in the chocolate chips, making sure they're evenly distributed. The dough is quite soft. Let it rest for a couple of minutes and it will firm up slightly.
Now divide the dough equally among the ramekins, smoothen the tops and bake for 20 minutes for gooey, slightly under-baked cookie pots (highly recommended!), or up to 25 minutes if you prefer browner tops and less gooey centers. If you like, sprinkle a few chocolate chips and some flaky sea salt on top, as soon as they're out of the oven.
Allow the ramekins to cool for 10 minutes. The centers of the cookies will fall slightly. Then add a scoop of ice cream to each and dig in! Best eaten fresh and warm, but you can store them in the fridge for 4 to 5 days and reheat as needed. Happy baking!