Instructions

• Preheat the oven to 350°F (175°C). Spray a 9-inch glass pie plate very well with cooking spray and set aside.
• In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric handheld mixer, beat the softened butter on low speed until smooth, about 1 minute.
• Add the light brown sugar and granulated sugar. Beat on medium-high speed until creamed and well combined, about 2 to 3 minutes. Stop and scrape down the sides of the bowl.
• Add the egg and vanilla extract. Beat on medium-high speed until the mixture is light and fluffy, about 2 minutes. Scrape down the sides and bottom of the bowl.
• Add the all-purpose flour, cornstarch, baking soda, and salt. Mix on low speed until just combined, about 30 seconds. Do not overmix.
• Add the mini semi-sweet chocolate chips and mix on low for a few seconds, or stir by hand, until evenly distributed.
• Gently fold in the mini marshmallows by hand or mix on low just a few seconds to incorporate. Reserve some marshmallows to place on top for visual appeal, and try to avoid pressing marshmallows against the sides or base of the pie plate to make cleanup easier.
• Transfer the dough to the prepared pie plate and smooth the top with a spatula so it is even, leaving some marshmallows visible on top.
• Bake for about 18 to 20 minutes, or until the center is just set and the marshmallows are just barely golden. The edges should be more set and lightly golden—do not overbake if you want a gooey center.
• Allow the pie to cool until it is comfortable to eat (warm or room temperature), then serve.