Heat oven to 350°F. Line 8-inch square pan with foil so edges extend over pan.
Lightly spoon flour into measuring cup; level off. In medium bowl, combine flour, brown sugar and cornstarch; mix well. With pastry blender or fork, cut in margarine until mixture resembles coarse crumbs. Stir in hazelnuts and vanilla. Press evenly in bottom of foil-lined pan.
Bake at 350°F. for 20 to 25 minutes or until golden brown. Cool in pan 2 minutes.
Remove bars from pan by lifting foil. Immediately cut into 6 strips. Cut once diagonally from 1 corner to opposite corner. Make 3 parallel cuts on each side of diagonal cut, forming diamond shapes. (Some end pieces will be incomplete diamonds.) Cool on foil for 20 minutes or until completely cooled.
Meanwhile, in small saucepan, melt chocolate over low heat, stirring constantly until smooth. Dip half of each pecan half in melted chocolate; place on waxed paper-lined cookie sheet. Refrigerate 5 minutes or until chocolate is set.
In another small saucepan, combine caramels and water; cook over low heat until caramels are melted and mixture is smooth, stirring constantly.
To assemble bars, remove cooled bars from foil; place on waxed paper-lined cookie sheet. Spoon 1/2 teaspoon melted caramel in center of each bar. (If melted caramel runs off first bar, wait 1 to 2 minutes for caramel to thicken before continuing.) Top with 1 chocolate-coated pecan half.