Pre-heat oven to 350°. Combine first 3 ingredients in heavy saucepan; cook over medium-low heat, stirring constantly, until chocolate and butter melt. Remove from heat. Stir in vanilla, flour, and chopped pecans. Drop by teaspoonfuls onto ungreased baking sheets. Press a pecan half into the center of each cookie. Bake for 7 minutes. (Do not over-bake.) Remove cookies to wire racks to cool completely.