- Whisk the melted butter, brown sugar & granulated sugar together in a large bowl. 
- Whisk in the egg and egg yolk, along with the vanilla extract. 
- Add in the flour, cornstarch, baking soda, and salt, and mix until combined. 
- Fold in half of the Reese's Pieces and all of the white chocolate chips. 
- Cover and chill in the refrigerator for a minimum of 3 hours – up to 3 days Overnight works best. 
- Remove the dough from the refrigerator and let it soften at room temperature while you preheat the oven. 
- Preheat the oven to 325 degrees. 
- Line the baking sheets with parchment. 
- 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 and they will 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.