Place the butter in a medium saucepan and set it over medium heat.
Pour the butter into a large mixing bowl or the bowl of a stand mixer and set aside to cool.
Assemble the Dough
While the butter cools, whisk the flour, salt and baking soda together in a medium bowl.
When the butter is cool enough, add both sugars, the vanilla extract and the egg to the large bowl with the butter, and mix until combined.
Add the flour mixture and mix until just combined.
Fold in the chocolate chunks until they are evenly distributed.
Shape the Cookie Dough
Scoop the dough into large balls with an ice cream scoop and use your hands to mold it into round and flat discs, like small hockey pucks.
Bake
Bake the cookies at 350°F (177°C/gas mark 4) for 10-11 minutes. If you notice even a small amount of browning at the bottom, they are probably done. They will still be very soft when you take them out, but will firm up quickly. The internal temperature of the cookies will be 175-180*F when they are done.
Store
These are best on the day they are baked, but if you have any leftovers, you can keep them at room temperature for a few days.
Make Ahead
If you want to make these ahead, the best way is to freeze the actual cookie dough before baking it. After you shape it into pucks, pop your cookie sheet in the freezer until the dough firms up a little bit, and then put the pucks into an airtight freezer container.
You can bake these directly from frozen, but they will not spread as much and come out much smaller (see picture: cookie baked directly from frozen is on the left). If you're baking from frozen, bake them at 325°F (165°C/gas mark 3) for 15 minutes. You can also defrost them in the refrigerator and let the dough come to room temperature before baking, in which case you can bake them normally.