Combine first 5 ingredients in a large heavy saucepan. Cook over medium heat, stirring constantly until mixture comes to a boil; boil 5 minutes, stirring constantly. Remove from heat. Add chocolate morsels to marshmallow mixture, stirring until chocolate melts. Add pecans and vanilla, stirring well. Spread evenly in a buttered 8 or 9 inch square pan. Cool and cut into squares.
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