In a large bowl, whisk together the tahini, melted butter, brown sugar, and granulated sugar until smooth and combined. Add the egg and vanilla extract and whisk vigorously until the mixture is thick and slightly glossy.
Add the flour, baking soda, and salt to the bowl. Fold with a spatula until just combined with no dry streaks remaining. Fold in the chopped dark chocolate.
Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes. This step firms up the dough and develops the flavors, making a huge difference in the final cookie.
Preheat your oven to 375F. Line a baking sheet with parchment paper. Scoop the dough into balls of about 2 tablespoons each and place them 2 inches apart on the prepared sheet. Press a few extra chocolate chunks into the tops if desired.
Bake for 10 to 12 minutes, until the edges are golden but the centers still look slightly underdone. They will firm up as they cool.
Immediately sprinkle flaky sea salt over the warm cookies. Let them cool on the baking sheet for 10 minutes before transferring to a wire rack.