8 ounces unsalted butter (2 sticks; 225g), soft but cool, about 65°F (18°C), plus more for greasing skillet
4 1/2 ounces white sugar (2/3 cup; 125g)
8 ounces light brown sugar (1 cup, gently packed; 225g)
1 ounce malted milk powder (about 1/4 cup; 25g); see note
1/2 ounce vanilla extract (1 tablespoon; 15g)
2 teaspoons (8g) Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
1 teaspoon baking soda
1/2 teaspoon baking powder
1/8 teaspoon grated nutmeg
1 large egg (about 1 3/4 ounces; 50g), straight from the fridge
12 1/2 ounces all-purpose flour, such as Gold Medal (2 3/4 cups, spooned; 355g)
12 ounces assorted dark, milk, or white chocolate (not commercial chips), roughly chopped (about 2 cups; 395g)
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.
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.