1. Preheat your oven to 350°F. Line a baking sheet with parchment paper.

2. In a large bowl, cream together the softened butter, peanut butter, granulated sugar, and brown sugar until light and fluffy. Add the egg and vanilla extract, mixing until well combined.

3. In a separate bowl, whisk together the all-purpose flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until a soft dough forms.

4. Roll the dough into 1-inch balls and shape each one into an oval. Place on the prepared baking sheet, spacing apart. Use a fork to press a crisscross pattern onto each cookie and sprinkle lightly with extra granulated sugar.

5. Bake for 8–10 minutes, or until edges are lightly golden. Let cool on the baking sheet for 5 minutes before transferring to a wire rack.

6. Prepare the filling by beating together the peanut butter and softened butter until smooth. Add the powdered sugar and vanilla extract, mixing until creamy. If needed, add milk to achieve desired consistency.

7. Once cookies are cooled, spread or pipe the filling onto one cookie and top with another to form a sandwich. Repeat with remaining cookies and filling.