1. In heavy 3-quart saucepan over medium heat, heat sugars and milk to boiling, stirring constantly until sugars are completely dissolved. Set candy thermometer in place and continue cooking, without stirring, until temperature reaches 238°F., or soft-ball stage (when small amount of mixture dropped into very cold water forms soft ball that flattens on removal from water).
2. Remove saucepan from heat; add butter or margarine and vanilla but do not stir. Cool mixture to 110°F or until outside of saucepan feels lukewarm. Meanwhile, lightly butter 8" by 8" baking pan.
3. When mixture is lukewarm, with wooden spoon, beat until it becomes thick and begins to lose its gloss. Quickly stir in pecans; pour into pan. (Don't scrape saucepan; mixture on side may be grainy.)
4. Cool candy in pan on wire rack. When cold, cut into 6 strips, then cut each strip crosswise into 6 pieces. Store tightly covered.