Prepare 9-inch square pan by lightly buttering it or lining with parchment paper.
In heavy 2 to 3-quart saucepan, melt butter over medium heat. Add sugar, corn syrup and sweetened condensed milk, stirring constantly.
Cook over medium heat, mixture will caramelize to a golden brown and come to a boil.
Clip on candy thermometer and continue to cook over medium heat stirring frequently to prevent scorching until the temperature of the mixture reaches 248°F (firm ball stage.)
Pour mixture evenly into prepared pan. When the candy is firm, cut into ¾ inch strips and allow to finish cooling.
Turn candy out of pan and cut into ¾ inch squares.
Wrap individual pieces in 2¾ to 3-inch squares of waxed paper.
Store in an airtight container.
Do Not Use waxed paper, plastic wrap or foil to line the pan. Foil adheres to toffee and is difficult to remove; waxed paper and plastic and plastic wrap will not withstand the heat of the candy.
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