In a stand mixer fitted with a paddle attachment, beat together the butter, brown sugar, and granulated sugar until smooth and creamy.
Beat in the eggs, one at a time, followed by the vanilla extract.
Pulse in the dark cocoa powder until incorporated, then mix in the all-purpose and cake flours with the baking powder, cornstarch, and salt until just combined.
Fold in the chocolate chips, marshmallows, and walnuts.
Use a standard ice cream scoop or pull off a 1/4 cup piece of dough. Roll into a ball and place on a parchment-lined baking sheet. Press down lightly. Repeat with the remaining dough, dividing them between two baking sheets and about 2-3 inches apart. If desired, top the cookie balls with additional chocolate chips and marshmallows.
Place the baking sheets in the freezer for 1 hour.
Preheat oven to 350˚F. Bake the frozen cookies in the preheated oven until just set, about 15-18 minutes. Allow to cool to room temperature, about 20 minutes, to set. They are best the day they are baked.