In a large bowl, use electric mixer to cream together butter and brown sugar. Add vanilla. Beat in flour, 1 cup at a time, until it is mixed well. Beat in sweetened condensed milk and mix until well-combined. Stir in 1 cup of chocolate chips.
Cover bowl and refrigerate until dough is firm (at least 1 hour). Use cookie scoop to scoop out heaping tablespoonfuls, roll into balls and place on a waxed paper-lined cookie sheet. Place all of the rolled balls back into the refrigerator until you are ready to dip them in chocolate.