Makes about 14 large cookies.
Preheat your oven to 350 degrees F. Line a baking sheet with parchment paper. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, combine the butter, peanut butter, brown sugar, milk and vanilla. Beat on medium speed until well blended, about one minute.
Add the egg and beat until just combined.
Add the flour, baking soda and salt and mix on low speed until just combined.
Roll cookie dough balls, about the size of golf balls. Place 12 on a cookie sheet. Using the back of a fork, press gently into the cookie and then again the opposite direction.
Bake for 9 to 10 minutes, or until slightly golden brown on top. Do not over bake. In fact, I like these underdone just a little. Let the cookies cool completely before adding buttercream.
You should have 28 cookies to make 14 sandwich cookies.