Beat brown sugar and butter together in a large bowl using an electric mixer until smooth and creamy; add vanilla extract and beat until smooth. Gradually beat flour into creamed butter mixture until smooth; add sweetened condensed milk and beat until smooth. Fold chocolate chips and pecans into cookie dough mixture; shape into 1-inch balls and place on prepared baking sheets. Refrigerate cookie dough balls until chilled, about 2 hours.
Melt chocolate bark in the top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. Dip each cookie dough ball into melted chocolate using 2 forks to hold the ball. Place dipped ball onto the waxed paper and repeat with remaining balls and chocolate. Chill in refrigerator until set, at least 1 hour.
