Mix all the dry ingredients and set aside.
Using a stand mixer fitted with the paddle attachment, beat the melted butter and two sugars until smooth, about 1 minute.
Add the eggs, egg yolk, and vanilla extract and beat for another minute. Scrape down the bowl as needed.
using a spatula, gradually mix in the dry ingredients until you don't see any trace of flour.
Add in the chopped milk chocolate, semisweet chocolate, and the Reese pieces, and gently mix.
Cover the dough and chill for 30 minutes. Meanwhile, preheat the oven to 385°F
Scoop the batter into small balls, the size of a small tangerine or ¼ cup.
Bake for 8-10 minutes. until the edges start to set and brown. Leave in the tray for 5 minutes then transfer to a cooling rack.