Line a 8 x 8 x 2 inch square pan with aluminum foil. Butter the pan with 1 tablespoon of the butter. Butter the bottom and sides of a 2 quart heavy saucepan with 1 teaspoon of the butter. Combine the sugars, light cream, chocolate and corn syrup in the saucepan. Place over medium heat and cook until the mixture reaches a boil, stirring constantly. Carefully clip a candy thermometer to the side of the pan. Continue to cook, stirring constantly, until the mixture reaches the soft ball stage, (234º).

Remove from the heat and add the butter and vanilla. Let the mixture stand, without stirring, for about 1 hour or until the mixture reaches 110º.