Preheat the oven to 350 F. In a medium bowl, whisk together flour, oats, salt, and baking soda. In a large bowl, mix together butter, peanut butter, brown sugar, and granulated sugar until fluffy.
Add in the egg and vanilla extract and mix until it is fully smooth. Add in the flour mixture and mix until it is fully combined. Fold in the Reese's pieces.
Press the cookie dough into a greased round 9-inch pie plate. Bake for 20-25 minutes.