Warm your oven to 410°F and prepare a baking sheet with parchment paper.
In a large bowl, whip the cold butter with brown and white sugars until the mixture is pale and fluffy, about 4 minutes.
Add eggs and vanilla extract, beating together for another minute to combine fully.
Gently stir in cocoa powder, cake flour, all-purpose flour, cornstarch, baking soda, and salt until dough comes together.
Fold in the chocolate chips, mini marshmallows, and chopped cashews evenly throughout the dough.
Shape the dough into large balls weighing about 5 to 6 ounces each and arrange them spaced out on the lined baking sheet.
For a decorative touch, press a few extra chocolate chips and marshmallows lightly onto the tops of each cookie.
Bake the cookies for 9 to 11 minutes until edges look set but centers remain soft.
Right after baking, use a small spatula to gently push the edges of each cookie towards the center to maintain their shape and prevent melting marshmallows from spreading.
Allow the cookies to cool on the baking sheet for 15 minutes so they firm up before transferring them.
