Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the melted salted butter, packed brown sugar, and granulated sugar. Whisk together until smooth. Add in the egg, egg yolk, and vanilla extract. Continue to whisk until everything is well combined.
In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt.
Gradually add the dry ingredient mixture into the wet ingredients. Stir gently until just combined.
Gently fold in the milk chocolate chips and half of the Reese's Pieces candies. Reserve the other half for topping the cookies later.
Using a cookie scoop, portion out the dough onto a lined baking sheet, leaving enough space between each cookie to allow for spreading.
Gently press a few of the reserved Reese's Pieces into the top of each cookie.
Bake the cookies for 10-12 minutes or until the edges are lightly golden.
Remove from the oven and let them cool on the baking sheet for about 5 minutes before transferring to a cooling rack.
