Preheat oven to 350°. Line a cookie sheet with parchment paper and set aside.
Add the butter, oil and both sugars to a medium mixing bowl. With an electric mixer, cream together until smooth and creamy.
Add the egg and beat until incorporated. Then add the vanilla.
Combine the flour, baking soda, baking powder and salt together in a bowl.
Add half of the dry ingredients to the creamed mixture and stir until just combined. Add the remaining half and stir until all of the ingredients are wet.
Add the chocolate chips and Reeses Pieces and combine. Cover and chill dough for 15 minutes.
Once chilled, remove from refrigerator. Using a cookie scoop, add the cookie dough on the lined cookie sheet, 2 inches apart.
Bake for 10 to 12 minutes (depending on oven temperatures). Remove from oven and allow the cookies to cool on the pan for 2-3 minutes.
Transfer the cookies to a wire rack to completely cool before serving.