Add butter to a 12-inch cast-iron skillet and melt the butter while the oven preheats.
Meanwhile, in a large bowl, stir together brown sugar and granulated sugar.
When butter is almost melted, carefully remove skillet from oven and whisk butter until completely melted.
Pour the butter over the sugar mixture and whisk until well-combined.
Let the butter mixture rest for 5 minutes.
Mix in eggs and vanilla.
Add flour, baking soda, and salt, and stir in just until combined.
Stir in chocolate chips.
Spread mixture into buttered skillet.
Bake until toothpick inserted in center comes out with a few moist crumbs and the top is golden brown, about 30-40 minutes.
Serve warm.
Notes
Butter: I use salted butter. If using unsalted butter, increase the salt to 3/4 teaspoon.
Brown sugar: Light or dark brown sugar will work. Dark brown sugar will give you richer flavor.
All-purpose flour: It's important to properly measure the flour. Weigh it for the most accurate measurements. You can also sift/stir it to break it up, lightly spoon it into the measuring cup, and level.