Preheat your oven to 350℉. Grease a 10-inch cast-iron skillet with butter or cooking spray.
Add butter, both sugars, and tahini to your stand mixer bowl and fit with a paddle attachment. Beat on medium speed until fluffy, 3 to 4 minutes. Scrape the sides of the bowl, then beat in eggs one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flour, baking soda, and salt. With mixer on low speed, gradually add the flour mixture, beating until combined. Shut off the mixer and fold the chocolate and cranberries into the dough gently but thoroughly with a spatula. Spread the dough in the prepared skillet.
Bake for 35 to 40 minutes. Remove from the oven and immediately sprinkle with sea salt. Let cool slightly, or cool to room temperature. You can serve it in warm scoops with ice cream, or sliced in neat wedges or chunks after it cools to room temperature.