Line two baking sheets with parchment paper or silicone baking mats.

In a large bowl, whisk the dry ingredients together and set aside.

Using a stand mixer with the paddle attachment, beat the butter, peanut butter, brown sugar and granulated sugar on medium-high speed until the mixture is light and fluffy, about 3–4 minutes, scraping down the sides of the bowl as needed.

Add the egg, egg yolk, milk and vanilla, and mix until fully combined, scraping down the sides of the bowl as needed.

Gradually add in the dry ingredients, mixing just until combined. The dough should be light, fluffy and airy but not overly sticky.

Gently fold in the Reese’s Pieces and mini chocolate chips using a spatula, taking care not to over-mix.

Allow the dough to rest for 20 minutes at room temperature or in the fridge. This short resting time lets the flour hydrate, resulting in soft and chewy cookies.

Preheat the oven to 350°F. Using a 2-tablespoon scoop, portion the cookie dough (makes about 24 cookies) and place them on the prepared baking sheets. Gently press a few extra Reese’s Pieces onto the tops of each cookie.

Bake for 9-11 minutes, keeping them slightly underbaked for a perfectly chewy texture. The edges should be light golden brown, and the centers should look slightly gooey—they will finish baking on the sheet after being removed from the oven. If you’d like to make them into perfect circles, use a cup to gently swirl around the warm cookies.