• Preheat oven to 350 degrees and line a baking sheet with parchment paper. Set aside.
• Mix together the peanut butter, sugar and egg. Use a tablespoon portion scoop to scoop the cookie dough into a ball. Divide that ball in half and roll into two smaller balls. Place the balls next to each other on the baking sheet and use a fork to press the cookies flat, pressing the fork in both directions to create that classic crosshatch shape.
• Bake for 8 to 10 minutes or until golden brown on the bottom. Remove from the oven and let the cookies cool completely before adding the filling. If you need to move the cookies while they are still hot, be careful, since the two balls can separate from each other. Leave them on the baking sheet until they cool, or if you need to move them to a cooling rack, handle them gently. Once they cool they hold together fine.

Make the Filling

• Using a mixer, add the peanut butter, powdered sugar and 4 tablespoons of milk. Beat until smooth. If needed, add the additional milk a little at a time until you reach a consistency that is easy to pipe onto the cookies.
• Place the filling into a piping bag and cut a 1/2 inch tip.
• Place one cookie upside down onto a flat surface and pipe the filling into the center of the cookie. Gently press another cookie, bottom side down, onto the filling to make a sandwich. Repeat until all the cookies are sandwiched together.