Cream peanut butter, butter, and sugars together in a mixing bowl.
Beat in egg, vanilla, and milk and continue mixing until well combined.
In a separate bowl, whisk together flour, corn starch, baking soda, baking powder, and salt.
Gradually add flour mixture to peanut butter mixture, mixing until dough comes together.
Refrigerate for at least 2 hours.
Preheat oven to 350F.
Scoop out dough using a cookie scoop. (You want about two tablespoons of dough.) Use your hands to form into a ball.
Roll balls in remaining 1/4 cup of sugar and place on parchment line cookie sheet. Use a fork to press down on the cookie, flattening it slightly.
Bake for 9-12 minutes, just until cookie is set. I like mine slightly underdone but you can go the fulll 12 minutes if you like.
Remove cookies and cool for 5 minutes on cookie sheet before moving to a cooling rack. Let cookies cool completely.