In a stand mixer, beat cold cubed butter, brown sugar, and granulated sugar on medium-high for 3–4 minutes until light and fluffy. Mix in eggs one at a time, then vanilla, scraping down the bowl as needed.
In a medium bowl, whisk together flour, cocoa powder, baking soda, baking powder, salt, and espresso powder.
Add dry ingredients in two additions, mixing on low speed until just combined.
Fold in chocolate chips, chopped chocolate, roasted nuts, and mini marshmallows (reserving a few for topping).
Use a large cookie scoop to scoop dough into large ⅓-cup balls and press extra marshmallows and chocolate chunks onto tops. Chill at least 2 hours or overnight for best results. Preheat oven to 400°F (204°C). Place dough balls on parchment-lined baking sheets 3 inches apart. Bake 10–12 minutes until edges are set and tops are crackled but centers are still slightly underbaked. Cool on the baking sheet for 10 minutes. Sprinkle with flaky sea salt before serving.
