(The hot caramel mixture needs to be poured immediately after stirring in vanilla, so be sure to butter your pan ahead of time.)
Bring the sugar, dark corn syrup and butter to a boil in a 3-qt. saucepan over medium heat; cook, without stirring, 7 minutes. Stir in condensed milk, and bring to a boil over medium heat; cook, stirring constantly, until a candy thermometer registers 238 degrees to 240 degrees (soft ball stage), about 20-25 minutes. Remove from heat, and stir in vanilla. Pour into a buttered 8-inch square pan. Let stand at room temperature 8 hours. Invert caramel onto a cutting board; cut into 1-inch squares using bench scraper or knife. Cook chocolate and shortening in a saucepan over medium heat 3 to 5 minutes until melted and smooth. Remove from heat.
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