Preheat oven to 350°F/175C. Line baking sheets with parchment.
Whisk together flour, cornstarch, baking soda, baking powder, and salt. Set aside.
In a large bowl, combine oil and peanut butter. Beat until well blended with a hand mixer or a whisk.
Add sugars and beat until smooth. Add oat milk and vanilla extract. Beat until smooth.
Add the flour mixture to wet ingredients, mix until dry ingredients are just incorporated.
Scoop slightly heaping 1 tablespoon (18ml) portions of dough. Roll into balls and place on lined baking sheets with 2-3 (/5-8cm) between them.
Press on each cookie with a fork in a criss-cross pattern for the classic design. Flatten the cookies to about ½” (12mm) thick. For chewier cookies flatten less.
Bake for 10-12 minutes until firm on the edges. Allow to cool on the baking tray.
Store at room temperature for 5 days or in the freezer for up to 1 month.
Notes
For chewier cookies, do not press the criss-cross pattern as deeply.
Add a bit of extra texture easily by using crunchy peanut butter in place of smooth.
The cookies will spread a bit, so leave 2-3 inches between each cookie so that they have room to bake.
Dip the fork in sugar to add a sugary finish on top of the cookies or sprinkle with flaky salt for a sweet and salty finish.