Heat oven to 350°F. Line two baking sheets with parchment.
Cream butter, granulated sugar, and brown sugar until light and fluffy.
Beat in egg and vanilla until smooth.
Whisk flour, baking powder, baking soda, salt, and cinnamon in a bowl.
Mix dry ingredients into the butter mixture just until combined. Add milk only if dough is crumbly.
Stir rolling sugar and cinnamon together in a small bowl.
Scoop tablespoon portions, roll into balls, and coat in cinnamon sugar.
Arrange 2 inches apart on sheets. Bake 10–12 minutes, until edges set and tops crackle.
Cool 5 minutes on sheet, then move to a rack to finish cooling.