Pre heat oven to 350°
Cream together the softened butter and sugars.
To the butter sugar mixture, add in egg, vanilla and peanut butter. Mix thoroughly until combined.
Whisk the dry ingredients together in another bowl.
Slowly add in the dry ingredients to the wet ingredients. Mix until thoroughly combined.
Refrigerate dough for 30 minutes.
Roll into large balls and place on a baking sheet.
You should be able to get 6 balls to a cookie sheet, making 12 cookies total. Flatten the balls of dough slightly to ensure that they bake flat.
Bake for 18 minutes.
Allow to cool completely and make the frosting in the mean time.
In the mixer, combine powdered sugar, peanut butter and vanilla.
While mixing, slowly add in one tablespoon of oat milk at a time until you get the desired consistency.
Mix at high speed for 2 minutes until the frosting is whipped and fluffy.
Spread frosting on bottom of cookies and smush together to make the sandwich.