Whisk the melted butter, brown sugar & granulated sugar together in a large bowl.
Whisk in egg & egg yolk, along with the vanilla extract.
Add in the flour, baking soda, cornstarch & salt & mix until combined.
Fold in the white chocolate chips & 1/2 cup of Reese's Pieces.
Cover & chill in the refrigerator for a minimum of 3 hours – up to 3 days Overnight works best.
Remove the dough from the refrigerator & let it soften at room temperature while you preheat the oven.
Preheat the oven to 325 degrees.
Line your baking sheets with parchment & set aside.
Roll the dough into large, oblong-shaped balls (taller than they are wide) – approx. 3 tbsp each. The dough will be crumbly but will come together when rolled between your hands.
Place the dough on your prepared baking sheet, approx. 1-2” apart.
Place Reese’s Pieces into the top of the dough before baking.
Bake 11-12 minutes – they will look very soft but will continue to bake once removed & they set.
Place any additional Reese’s Pieces or chocolate chips in the tops of the baked cookies before they cool, if desired.
Cool on the cookie sheet for 10 minutes before transferring to a wire rack to cool completely.