Melt butter and chocolate chips in a 2-quart saucepan over low heat, stirring constantly, until smooth, about 5 to 10 minutes. Remove from heat. Add condensed milk; stir until smooth. Add flour; mix well. Shape rounded teaspoonfuls of dough into 1-inch balls. Place 1 inch apart onto un-greased cookie sheets.
Bake for 6 to 8 minutes or just until set, do not overbake. Cookies may appear slightly under baked. Cool completely. Meanwhile, combine powered sugar and vanilla in small bowl. Gradually stir in enough milk for desired glazing consistency. Drizzle over cookies.