In a small bowl, melt the butter and cool in the fridge for about 15 minutes.
1 cup (226 g) unsalted butter
In another bowl whisk together the flour, baking powder, baking soda, cornstarch and salt until well combined.
2 cups (240 g) all purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt
In a large mixing bowl, whisk in the cooled melted butter with the light brown and granulated sugars until well combined. Whisk in the egg, egg yolk, vanilla extract and cocoa powder until smooth and well combined. Stir the dry ingredients into the wet until just combined. 1 cup (200 g) granulated sugar, ½ cup (100 g) light brown sugar, 1 large egg, room temperature, 1 large egg yolk, room temperature, 1 teaspoon (5 ml) vanilla extract , ½ cup (40 g) dutch process or unsweetened cooca powder
Preheat the oven to 350 F (177 C) for about 20-30 minutes before baking the cookies. Line a large baking sheet with parchment paper. Allow the dough to rest while the oven preheats for 30 minutes or transfer to the fridge to store for longer. Pour the sugar for rolling into a small bowl and set aside. Using a large cookie scoop (3 tablespoons, 45 ml), scoop the chocolate sugar cookie dough into the palm of a clean hand. Gently press the dough into a disk. Insert 6 - 7 mini marshmallows or one regular sized marshmallow into the center. Gently lift the sides up and around the marshmallows.
¾ cup (45 g) mini marshmallows or 15 regular sized marshmallows
Roll the cookie dough ball into sugar and place the cookie dough balls onto the lined baking sheet about 3 inches (8 cm) apart. Bake at 350 F (177 C) for 12 - 14 minutes or until the edges of the cookies are set and the centers are slightly puffed and some cracks have formed across the top of the cookie. ¼ cup (50 g) granulated sugar for rolling
Cool the chocolate mini marshmallow cookies on the baking sheet for 5 minutes. Then transfer to a cooling rack to cool completely to room temperature and enjoy!