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