Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a medium bowl, combine all of the dry ingredients: rolled oats, flour, baking soda and salt. Stir to combine and set aside.
In a large bowl, add the melted butter and brown sugar and whisk to combine. Then add the egg, peanut butter and vanilla extract and whisk again until well combined.
Add the dry ingredients to the wet ingredients and, using a spatula or a spoon, mix until well incorporated.
Portion the dough into 1 tablespoon-sized balls, using a scoop or your hands, place them to the baking sheet, leaving 2″ between them, and flatten them gently.
Transfer the baking sheet to the oven and allow to cook for 10-12 minutes until the edges are slightly golden and the centre still looks soft.
Once cooked, remove the baking sheet from oven and allow the cookies to cool completely on the baking sheet.
Once cooled, the cookies can be served immediately, stored in an airtight container for up to 3 days or in the freezer for up to 3 months.