Preheat oven to 375 degrees F. Line countertop with sheets of foil for cooling cookies.
In a large bowl, mix brown sugar, JIF® peanut butter, shortening, milk, and vanilla until well blended using an electric mixer on medium speed. Add the egg and beat until just blended.
Mix together the flour, baking soda, and salt. Slowly add to the creamed mixture while mixing at low speed. Stir just until combined.
Spoon the dough onto a baking sheet, 2 inches apart, using a rounded tablespoon. Gently press down with a fork in a crisscross pattern.
Bake each baking sheet for 7-8 minutes, or until edges are set and lightly golden. Avoid over-baking. Allow the cookies to cool on the baking sheet for 2 minutes, then transfer to foil to cool completely.