Chocolate Chip Skillet Cookie
* 8 oz. unsalted butter (2 sticks; 225 grams) , soft but cool, about 65◦F (18◦C), plus more for greasing skillet
* 4 ½ oz. white sugar (2/3 cup; 125 grams)
* 8 oz. light brown sugar (1 cup, gently packed; 225 grams)
* 1 oz. malted milk powder (about ¼ cup; 25 grams)
* ½ oz. vanilla extract (1 tbsp; 15 grams)
* 2 tsps. Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
* 1 tsp. baking soda
* ½ tsp. baking powder
* 1/8 tsp. grated nutmeg
* 1 large egg (about 1 ¾ oz; 50 grams) straight from the fridge
* 12 ½ oz. all-purpose flour, such as Gold Medal (2 ¾ cups , spooned; 355 grams)
* 12 oz. assorted dark, milk or white chocolate (not commercial chips), roughly chopped (about 2 cups; 395 grams)
1) Adjust oven rack to middle position and preheat to 400°F (200°C). Combine butter, white sugar, brown sugar, malted milk powder, vanilla, salt, baking soda, baking powder, and nutmeg in the bowl of a stand mixer fitted with a paddle attachment. Mix on low to moisten, then increase to medium and beat until light and fluffy, about 5 minutes. With mixer running, add egg and continue beating only until smooth. Reduce speed to low and add flour all at once, followed by chopped chocolate, then mix to form a stiff dough.
2) Generously butter a 10-inch cast iron skillet, then fill with cookie dough and press into an even layer. Bake until puffed and golden brown, about 35 minutes. Cool until crumb is set, about 45 minutes, before serving. Enjoy warm or cover with foil and store up to 3 days at room temperature.