Cook first 4 ingredients in a heavy saucepan over medium-low heat, stirring constantly, until morsels and butter melt. Remove from heat. Stir in vanilla, flour, and chopped pecans. Drop by rounded teaspoonfuls onto ungreased baking sheets. Press a pecan half into center of each mound.
Bake at 350° for 5 minutes. (Do not overbake.) Remove to wire racks to cool.