Heat your oven to 375°F.
Brown the butter in your cast iron skillet.
½ cup (1 stick or 113 grams) unsalted butter
While the butter is browning, sift or whisk together the flour, baking powder, baking soda, and salt into a small bowl.
1 cup (125 grams) all-purpose flour, ¼ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
Add the sugars to a mixing bowl or the bowl of your stand mixer and mix briefly to combine.
2 tablespoons (25 grams) granulated sugar, ½ cup ( 96 grams) dark brown sugar
When the butter has browned - it is better to under-cook it here - add it to the bowl with the sugars and let it cool slightly before beating together.
Add the egg and then the vanilla, beating about one minute between each.
1 large egg, 2 teaspoons vanilla extract
Add the dry ingredients and mix only until just combined.
Add the chocolate chips by hand and spoon the batter into your cast iron skillet, being careful if it is still warm.
1 cup (170 grams) chocolate chips
Bake for 16 to 20 minutes, until the skillet cookie is lightly browned, slightly firm to touch, and just starting to pull away from the sides of your cast iron skillet.
Serve warm with ice cream or cool and store at room temperature in an air-tight container.