NOTE: For the best results, chill the dough for at least 3 hours.
In a large mixing bowl, beat together peanut butter, butter, brown sugar, milk, and vanilla extract until well blended. Beat in the egg until well blended.
In a small bowl, whisk together flour, salt and baking soda.
Gradually add dry ingredients to the creamed mixture until well blended.
Transfer to a bowl and cover with plastic wrap.
Place in refrigerator to chill for 3 to 4 hours or overnight.
Preheat oven to 350 degrees F and line baking sheets with parchment paper or a silicone mat. Remove the cookie dough from the refrigerator.
Roll the dough into 36 to 40 balls and roll them in granulated sugar.
Place the cookie dough balls on a baking sheet.
Using a fork dipped in water, make a crisscross on the cookies while gently pressing down.
Bake for 8-10 minutes or until lightly golden brown.
Remove from oven and cool completely on a wire rack.