In a medium-small bowl, mix the wet ingredients until combined.
In another medium-small bowl, mix the dry ingredients, except the peanut butter cup pieces, until combined.
Carefully fold the dry ingredients and 3/4 of the peanut butter cup pieces into the wet ingredients until just combined.
Form the dough into a ball and gently flatten onto a metal baking sheet. Push the remaining peanut butter cup pieces into the top of the cookie dough.
Heat the oven to 350 degrees.
Place the baking sheet with the dough into the fridge and chill for 30 minutes.
Once the oven is preheated, remove the chilled baking sheet with the dough from the fridge, and place the baking sheet directly onto the middle rack of the oven. (Upper 2/3 of the oven works too.) Bake for 12 minutes. The edges of the cookie will be light golden brown, and the center of the cookie will be a little gooey. DO NOT OVERBAKE. Cool the cookie on the baking sheet before enjoying warm.