Preheat oven to 350° F. Line a 8" square baking pan with parchment paper and spray with nonstick spray. Set aside.
In a medium mixing bowl, whisk together the flour, baking soda, cornstarch and salt. Set aside.
In the bowl of a stand mixer (or using a hand mixer) fitted with the whisk attachment, combine melted butter with peanut butter until smooth, about 1 minute. Add both sugars and whisk until well mixed. Switch to the paddle attachment and add vanilla and egg. Mix well, about 2 minutes.
With the mixer on low, slowly add dry ingredients. Using a rubber spatula or wooden spoon, add chocolate chips and 3/4 cup Reese's Pieces. Stir until incorporated. The dough should be soft and slightly crumbly (that's okay).
Place dough in prepared pan and press, smoothing out the top until dough is firm. Sprinkle remaining Reese's Pieces candy over the top and gently press into the dough.
Bake bars for 20 minutes. Cool in pan for 10 minutes, then transfer onto a cooling rack to cool completely. Cut and serve.
Store in an airtight container at room temperature for up to a week.