In a medium bowl, whisk together 2 1/4 cups all-purpose flour, 1 cup unsweetened cocoa powder, 1 teaspoon baking soda, and 1/2 teaspoon salt; set aside.

In a large bowl or stand mixer fitted with the paddle attachment, beat 1 cup unsalted butter, 1 cup packed light brown sugar, and 1/2 cup granulated sugar on medium speed until pale and fluffy, about 3–4 minutes.

Add 1 egg, 1 egg yolk, and 1 tablespoon vanilla extract; beat on medium-low until combined.

Reduce mixer speed to low; gradually add the dry ingredients and mix until just incorporated.

Stir in 2 cups mini marshmallows and 1 cup semisweet chocolate chips.

Use a 2-tablespoon cookie scoop to portion dough 2 inches apart on lined baking sheets.

Bake at 350°F for 9–10 minutes, until the tops are set and the centers look slightly underbaked.

Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.