Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Cream together the softened unsalted butter, creamy peanut butter, granulated sugar, and brown sugar until light and fluffy.

Add in the egg and vanilla extract and mix until fully incorporated.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add this to the peanut butter mixture, stirring gently.

Scoop even balls of dough onto the baking sheet and flatten each ball slightly.

Bake for 8-10 minutes until edges are lightly golden. Let cool for a few minutes before transferring to a wire rack.

In a mixing bowl, combine the creamy peanut butter, softened unsalted butter, and powdered sugar. Mix until smooth, adding milk as needed.

Spread or pipe the filling onto the flat side of one cookie and sandwich with another cookie, pressing down slightly.