1 cup (125 g) all-purpose flour
1 cup (43 g) Dutch-process cocoa powder
1 cup (32 g) cornstarch
2 Tbsp. Nonfat dry milk powder
1 tsp. Baking powder
1 cup. Ground cinnamon
1/2 tsp baking soda
1 cup (packed; 200 g) dark brown sugar
2 tbsp unsalted butter
1 tsp. Diamond Crystal or ½ tsp. Coarse kosher salt
2 large eggs
2 tsp. Vanilla extract
4 oz. High-quality milk chocolate (35%–45% cacao), coarsely chopped
3 Tbsp. (or more) marshmallow creme