8ozmilk chocolate², coarsely chopped(I used 5 Hershey bars)
Combine butter, sugars, and peanut butter in a large bowl and use an electric mixer (you can use a stand mixer instead) to beat until creamy and well-combined.
Add egg, egg yolk, and vanilla extract and stir well.
In separate, medium-sized bowl, whisk together flour, cornstarch, baking soda and salt.
Gradually, with mixer on low speed, stir flour mixture into peanut butter mixture, pausing occasionally to scrape down sides of bowl.
Use a spatula to stir in chocolate pieces.
Cover cookie dough with plastic wrap and transfer to refrigerator. Chill for at least 30 minutes.
Once dough is almost finished chilling, preheat oven to 350F/175C. Line cookie sheets with parchment paper and set aside.
Once cookie dough has chilled and oven has preheated, remove dough from fridge and scoop cookie dough into roughly 1 ½ Tablespoon-sized balls. If desired, gently roll into balls between your palms and then place onto prepared cookie sheets.³
Place cookies at least 1 ½ inches apart on prepared baking sheets and bake for about 12 minutes.
Allow cookies to cool completely on baking sheet before serving.
¹I don't recommend using a "natural" peanut butter (the kind that separates and you need to stir before using).²I love using milk chocolate because it complements the peanut butter so nicely, but you can use your favorite variety of chocolate -- semisweet, dark, or even white chocolate will work here.³The cookies don't need to be rolled, this just makes them look neater and more uniform.