Heat oven to 350°F. Line two baking sheets with parchment.
Cream butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes.
Beat in the eggs and vanilla until smooth and combined.
Whisk flour, cornstarch, baking soda, cinnamon, and salt in a bowl.
Mix dry ingredients into the butter mixture on low just until no streaks remain.
Fold in oats and raisins. Splash in milk if the dough feels dry.
Rest dough 10 minutes to hydrate the oats.
Scoop 2-tablespoon mounds, spacing about 2 inches apart on sheets.
Bake 10 to 12 minutes until edges set and centers look soft.
Cool 5 minutes on sheets, then move cookies to a rack to finish cooling.