Preheat the oven to 400°F.
Sift together flour, cornstarch, baking powder, baking soda, salt and cocoa powder together, and set aside.
Cream butter and sugar together until fluffy, at least two minutes.
Add egg, and mix thoroughly.
Add flour/cocoa powder mixture ⅓ at a time, mixing until just combined.
Fold peanut butter chips into batter.
Portion batter into 4 ~6-ounce balls of dough.
Bake for 7-9 minutes, or until the tops are just about to/beginning to set.
Remove and let cool for at least 10 minutes.