Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Cream together softened unsalted butter, creamy peanut butter, granulated sugar, and brown sugar until light and fluffy.
Add one large egg and a teaspoon of vanilla extract, mixing until combined.
In a separate bowl, whisk together all-purpose flour, baking soda, and salt. Gradually add to the wet ingredients until a soft dough forms.
Roll the dough into small balls, shape into ovals, and use a fork to create a crosshatch pattern. Sprinkle granulated sugar on top if desired.
Bake for about 8-10 minutes or until lightly golden around the edges.
For the filling, beat together creamy peanut butter and softened unsalted butter until smooth, then mix in powdered sugar and vanilla extract. Add milk as needed for texture.