Place your marshmallows in the freezer for at least 1 hour.
In a small saucepan over medium-low heat, melt the butter. Once melted, increase the heat to medium and let the butter begin to foam. Keep cooking and stirring with a spatula, until the butter begins to brown and gives off a nutty smell (2-3 minutes). Pour into the bowl of a stand mixer, making sure you scrape out all of the brown butter stuck at the bottom of the pan. Let cool.
Add the sugars, salt, nutmeg, vanilla, baking soda, and baking powder to the stand mixer and beat with the paddle attachment. Beat for about 5 minutes until it is light and fluffy. Add the egg + yolk, and beat until the batter is smooth
Add in the flour and beat until smooth, but don’t overmix. Fold in the chocolate chips, but keep a small handful to the side for later.
Form the dough into balls (about 2 ½ tablespoon each), and press each one flat, then place the marshmallow. Fold the dough over the marshmallow.
Place the cookie dough balls on a large plate or baking sheet lined with parchment paper. Chill for at least 1 hour or up to overnight. You can place in a freezer bag and freeze for later if you wish!
Heat oven to 350°F/180°C and line three baking trays with parchment paper.
Place the cookie dough balls on your baking sheet, leaving at least 2 inches between them. Bake the trays one at a time in the middle rack. Bake for 10-12 minutes for soft cookies, a minute or two longer if you want them crispier. Once out of the oven, place your extra chocolate chips on top. Allow the cookies to cool for 5-10 minutes before using a spatula to carefully take them off and place on a wire rack to cool completely.
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Just click on ‘Metric’ above the ingredients. Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Storage: keep the baked cookies in an airtight container for up to 5 days at room temperature. Freeze unbaked cookie dough balls, when ready to bake just bake for an extra minute or two.