Preparation Steps

• Line two sheet pans with parchment. Spread 1 cup of the mini marshmallows on a plate and place in the freezer for 10–15 minutes (this helps them hold their shape). Preheat the oven to 360°F (182°C).
• In a large bowl, whisk together the flour, cocoa powder, baking soda, baking powder, fine sea salt, and espresso powder until evenly combined.
• In a separate bowl, whisk the cooled melted butter with the dark brown sugar and granulated sugar until glossy and thick, about 1 minute. Whisk in the egg, egg yolk, and vanilla until smooth.
• Tip the dry mixture into the butter mixture and fold with a spatula just until no dry streaks remain. Stir in the chocolate chips. Cover and chill the dough for 15–20 minutes to firm slightly for shaping.
• Scoop dough into 18 portions (about 1½ tablespoons each). Flatten one portion in your palm, place 3–4 of the frozen marshmallows in the center, then wrap and pinch to seal. Set seam-side down on the prepared pans, spacing 2 inches apart. Repeat. Chill the filled dough balls on the pan for 10 minutes while the oven finishes heating.
• Bake one pan at a time for 8 minutes. Pull the rack out, quickly press 2–3 of the remaining (unfrozen) mini marshmallows onto the tops of each cookie, and add a few extra chocolate chips if you like. Rotate the pan and bake 2–4 minutes more, until edges look set and the tops are just matte with gooey marshmallow showing.
• Cool on the pan for 5 minutes, then move cookies to a rack. While warm, sprinkle lightly with flaky sea salt (optional). Repeat with the second pan.