Position a rack in the center of the oven and preheat to 375 degrees F. Melt the butter in a 10-inch cast iron skillet over medium heat, then set aside to cool slightly, 5 minutes.
Whisk the granulated sugar, brown sugar, eggs, and vanilla together in a large bowl until smooth. Stir in the melted butter. (Do not wipe out the skillet.) Whisk the flour, baking soda and salt together in another bowl. Stir the dry ingredients into the wet ingredients, mixing well with a wooden spoon until dough is homogeneous (do not overmix). Stir in the chocolate chips or chunks.
Use a rubber spatula to scrape the dough into the prepared skillet, gently spreading it to the edges and smoothing the top. Bake until the top is cracked and golden, the edges are brown and the center is still soft, about 40 minutes. Cool on a wire rack for 15 minutes. Cut into wedges and serve warm with vanilla ice cream.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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