Preheat oven to 350F and line two baking sheets with parchment paper. In a small bowl, whisk together the flour, baking soda, baking powder, cornstarch and salt; set aside. Using an electric mixer, beat the butter, brown sugar and peanut butter on medium speed until light and fluffy, about 3 to 4 minutes. Add the egg and vanilla and beat to combine.

Reduce the mixer speed to low and slowly add the flour mixture to the wet ingredients and mix until just combined. Using a rubber spatula, stir in the oats, chocolate chips, peanut butter chips, and Reese's Pieces candies by hand.

Measure out ¼ cup balls of dough and place them on the baking sheets, at least 2 inches apart. Flatten slightly with the palm of your hand. Bake until the edges are just starting to brown and the top still looks slightly undercooked, about 11 13 minutes.

Remove from the oven and cool for 5 minutes on the baking sheet before transferring them to a wire rack to cool completely. Cookies can be stored in an airtight container at room temperature for up to 4 days.