2 ½ teaspoons (8g) Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
1 teaspoon (5g) baking soda
½ teaspoon baking powder
⅛ teaspoon grated nutmeg
1 large egg (about 1 ¾ ounces; 50g), straight from the fridge
12 ½ ounces all-purpose flour(2 ¾ cups, spooned; 355g)
12 ounces assorted dark, milk, or white chocolate (not commercial chips), roughly chopped (about 2 cups; 340g)
Vanilla ice cream to serve (optional)
1. Adjust oven rack to middle position and preheat to 350°.
2. 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.
3. With mixer running, add egg and continue beating only until smooth.
4. Stop mixer and add flour all at once; quickly and carefully switch mixer speed between off setting and stir setting just until flour mixture is nearly combined into butter mixture.
5. Add chopped chocolate, then mix on low speed to form a stiff dough.
6. Generously butter a 10-inch cast iron skillet, then fill with cookie dough and press into an even layer.
7. Bake until slightly puffed, edges are set and golden brown, and center is just set but still feels soft underneath when touched, about 45 minutes, loosely covering cookie with aluminum foil after 20 to 25 minutes to prevent excess browning if needed.
8. Cool until crumb is set, about 1 hour, before serving.
9. Enjoy warm or cover with foil when cool and store up to 3 days at room temperature. Serve with vanilla ice cream, if desired.