Preheat oven to 350 degrees. Beat sweetened condensed milk, peanut butter, egg and vanilla in large bowl until smooth. Add biscuit mix; mix well. Can use an electric mixer this is exremely thick. Chill at least 1 hour. I usually leave the dough in the refrigerator overnight and bake off the next day.
Shape dough into 1 inch balls. Roll in sugar. Place 2 inches apart on an ungreased baking sheet. Flatten with fork in criss cross pattern.
Bake 6 to 8 minutes or until lightly browned (do not overbake. Cool