In a medium bowl, whisk together the dry ingredients. Set aside.
In the bowl of a stand mixer (or in a large bowl with an electric hand mixer), add the butter, coconut oil, brown sugar, and granulated sugar. Cream together until well-mixed, 2-3 minutes.
Add the eggs and vanilla extract. Beat to fully combine. With the mixer running on low, slowly add the dry ingredients in 3 additions, beating well after each addition.
Stir in chocolate chips, and Reese's Pieces. Cover and refrigerate the dough for at least 1 hour and preferably overnight.
When you're ready to bake the cookies, preheat the oven to 350F. Line two large, rimmed baking sheets with parchment paper or silicone liners. Scoop the dough and shape into heaping tablespoon-sized balls (I use a #40 cookie scoop).
Place the dough balls a couple inches apart on the prepared baking sheets. Top each dough ball with a few sprinkles before baking if desired.
Bake for 9-11 minutes, until the edges are set and the center is mostly set. Cool the cookies for 3-4 minutes on the pan, then transfer to a wire rack to cool completely. Enjoy!