Preheat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, cocoa, baking soda, and salt in a medium bowl.
Beat butter, peanut butter, granulated sugar, and brown sugar until light and creamy.
Mix in egg and vanilla until smooth and combined.
Add dry ingredients. Mix on low just until combined. Drizzle in milk if needed.
Fold in chocolate chips, if using.
Scoop 1½-tablespoon portions. Roll in sugar. Arrange 2 inches apart on sheets.
Flatten gently with a fork to make a crisscross pattern.
Bake 8 to 10 minutes until edges set and centers look soft.
Cool 5 minutes on the sheet, then transfer to a rack to finish cooling.