heat oven to 350 F and line baking sheets with parchment paper
whisk flour baking soda cornstarch and salt until uniform.
Mix and Bake
beat butter brown sugar and granulated sugar until light and fluffy
mix in peanut butter until smooth then mix in egg vanilla and milk if using
add dry ingredients and mix just until no dry flour remains then chill dough 20 minutes if very soft
scoop 1 1 2 tbsp portions and roll into balls then roll in sugar if using and place 2 inches apart
flatten each with a fork to make a crisscross pattern
bake until edges are set and centers look slightly underdone 9 to 11 minutes
cool 5 minutes on the sheet then transfer to a rack to cool completely.
Notes
1. Use creamy peanut butter for the most consistent texture; do not overbake for maximum chewiness; chill dough longer for thicker cookies; swap up to half the flour with whole wheat flour for a nuttier flavor; add 1 cup chocolate chips if desired