• Preheat oven to 350 degrees. Line cookie sheet with parchment paper or use a non stick cookie sheet.
• In a large bowl, beat the butter, brown sugar, granulated sugar and peanut butter until light and fluffy. Add in the egg and vanilla, mixing until combined.
• In a small bowl, whisk together the flour, baking soda and salt. Add half of the flour mixture into the wet mixture and beat until combined, add the remaining flour and mix again. Stir in the oats.
• Form balls 1 1/2 to 2 tbsp in size, or use a cookie scoop for better uniformly sized cookies. Place on a cookie sheet 2 inches apart. Bake for 8-10 minutes, just until the tops are set. Allow the cookies to cool for a few minutes before removing them from the cookie sheet. Cool completely before filling the cookies.

Prepare the filling

• Using a hand mixer or stand mixer, beat the butter for a few minutes until it has become light and fluffy. Slowly add in the powdered sugar, 1/2 cup at a time, and continue mixing in between each addition. The mixture will appear very crumbly at this point.
• Add in the vanilla and heavy cream and continue mixing until the texture changes and the icing has formed.

To assemble the cookies

• You can spread the icing onto a cookie backside and top it with another cookie. If you prefer a prettier look, fill a piping bag with the icing and use a 1M tip for a ruffled look or you can just cut the tip of the bag off.
• If you like to have the nuts around the middle, fill the cookie with icing, top it with the other cookie and roll the edges in a bowl of nuts. These taste great with or without the nuts so that is optional.