1)  Preheat oven to 350 degrees F.

2)  In heavy saucepan, cook while stirring constantly with wooden spoon:

1/4 cup butter (salted is fine, or unsalted plus a pinch of salt)
14 ounces sweetened condensed milk 
12 ounces semisweet chocolate chips

3)  Remove from heat and stir in:

1 tsp  pure vanilla extract 
1 C all-purpose flour 
1/2 cup pecans, chopped

4)  Drop by teaspoonfuls onto ungreased baking sheets. Press a pecan half into the center of each cookie. (You should yield about 50 - 60 cookies, so you will need that many pecan halves).   Bake for 7 minutes. (Do not overbake.) Remove cookies to wire racks to cool completely.