Add butter to a 12-inch cast-iron skillet and melt the butter while the oven preheats.
1 cup butter
Meanwhile, in a large bowl, stir together brown sugar and granulated sugar.
1 ¼ cups brown sugar, ¾ cup 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.
2 large eggs, 1 teaspoon vanilla extract
Add flour, baking soda, and salt, and stir in just until combined.
2 ½ cups all-purpose flour, 1 ½ teaspoons baking soda, ¼ teaspoon salt
Stir in chocolate chips.
1 cup semisweet 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.