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

In a large mixing bowl, blend the softened butter, peanut butter, brown sugar, and granulated sugar until pale and fluffy, approximately 2–3 minutes.

Add in the eggs individually, mixing well after each. Stir in the vanilla extract.

In another bowl, sift together the flour, baking soda, baking powder, and salt.

Gradually incorporate the dry ingredients into the butter mixture, mixing just until the dough comes together.

Scoop out portions of dough using a rounded teaspoon and shape into small ovals to resemble peanut shells.

Arrange on the prepared tray, spacing them evenly. Press gently with a fork either in a crisscross or lengthwise pattern to mimic the traditional nutter butter appearance.

Bake for 8 to 10 minutes, or until the edges begin to turn golden. Let the cookies rest on the tray for 5 minutes before transferring to a wire rack to cool completely.

In a clean bowl, beat the butter with the peanut butter until smooth and well combined.

Add the powdered sugar in increments, beating until light and fluffy.

Mix in milk, a tablespoon at a time, until the filling is soft and spreadable.

Once all cookies have cooled entirely, spread or pipe a generous amount of filling onto the underside of one cookie.

Place a second cookie on top, pressing gently to form a sandwich.

Keep the finished cookies in an airtight container at room temperature for up to five days.