Gooey Double Chocolate Marshmallow Cookies

0 Start by prepping your dry ingredients. In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined. Set that aside while we move on to the butter and sugars.
1 In a large mixing bowl (or bowl of a stand mixer with the paddle attachment), beat the room temperature butter, brown sugar, and granulated sugar. Use a hand mixer or stand mixer on medium speed and beat for about 1½ minutes until the mixture looks fluffy.
2 Add in the vanilla extract, egg, and egg yolk. Mix on low just until it’s all incorporated—don’t overmix. Scrape the sides and bottom of the bowl.
3 Slowly add your dry ingredients to the wet ingredients. Mix on low or stir in by hand until you don’t see dry streaks. 
4 Fold in the chocolate chips using a spatula.
5 Now for the fun part: scoop the dough using a large cookie scoop (about 3 tablespoons per cookie). Press your thumb into the center of each dough ball and tuck a marshmallow inside. Fold the dough around it, making sure it’s mostly sealed but a little marshmallow peeks through the top of each cookie.
6 Place the cookie dough balls on a parchment-lined baking sheet or cookie sheet and chill for at least 30 minutes. This helps keep the gooey marshmallows from melting out too early while baking.
7 Preheat your oven to 350°F. Prepare two sheet pans with parchment paper or non-stick spray. Space the chilled dough balls out about 2½ inches apart—these cookies spread a bit.
8 Bake for 10–12 minutes, or until the edges of the cookies are set and the centers look just slightly underdone.
9 Let the baked cookies cool completely on the cookie sheet before moving them to a wire rack. This helps them finish setting up without losing their gooey centers.