In a saucepan, combine the butter with sugar and milk. Keep stirring as the mixture comes to a boil, boil for 1 minute then remove from heat.
Stir in the peanut butter and vanilla until the peanut butter is melted and well combined in the mixture.
Stir in the oats, mix until well combined.
Spoon about 1 ½ tablespoons per cookie on a sheet pan lined with parchment paper or a silicone mat (I like to use a cookie scoop), you have to work quickly before the mixture sets. Gently press down with your fingers, and allow the cookies to harden.
Store. The cookies can be stored for up to 7 days in an airtight container at room temp.
Freeze. Freeze the cookies on a sheet pan for half an hour, then take them out and arrange in a container and freeze. These cookies are also great for shipping.
Gluten-free. For GF cookies, make sure that the oats you're using are gluten-free certified.
For the cookies to set and harden quicker, pop them in the fridge.