In a large bowl, stir together the flour, baking soda, cornstarch, and salt. Set aside.
In a medium bowl, whisk together the melted butter, brown sugar, and white sugar until no lumps are left.
Whisk in the egg, then the egg yolk.
Whisk in the vanilla.
Pour the wet ingredients into the dry ingredients and mix (I like to use a spatula to help scrape down the sides as I go). The dough will be very soft and buttery.
Fold in the chocolate chips and Reese's Pieces until they're evenly distributed.
Cover the dough and chill for at least 2 hours. I did mine overnight.
After chilling, set the dough out to soften to room temperature.
Meanwhile, preheat the oven to 325 degrees and line your baking sheets with silicone mats or parchment paper.
Roll the dough into balls, approximately 3 Tablespoons each. (I used my cookie scoop to help me)
[Make the dough balls taller rather than wider to end up with thick cookies]
They're large so you should have 8 per tray instead of the of the usual 12.
Press a couple more Reese's Pieces into the the top of the dough balls, if you want. They'll come out prettier that way.
Bake for 11-12 minutes. They'll look underdone but that's ok because you're going to leave them on the baking sheet for another 10 minutes to cool and they're continue baking.
After 10 minutes, transfer to a wire rack to cool completely.