BEAT brown sugar, peanut butter, shortening, egg, milk and vanilla in large bowl with mixer on high speed until smooth.
STIR flour, baking soda and salt in small bowl until blended. Gradually add to creamed mixture, beating just until blended. Roll into 36 (1 1/2-inch) balls. Place 2 inches apart on baking sheet. Flatten slightly with fork in a criss-cross pattern.
BAKE 7 to 8 minutes or until set and lightly browned. Cool 2 minutes on baking sheet. Remove to wire rack to cool completely.