2 sticks unsalted margarine
5 cups thick cut potato chips
1 cup sugar
2 teaspoons vanilla extract
2 tablespoons apple cider vinegar or white vinegar
5 ounces bittersweet or semisweet chocolate, finely chopped
Line a baking sheet with foil and spray with non-stick cooking spray. Set 1 cup potato chips aside. Break the remaining chips in half if they are large.
Combine the margarine, sugar, vanilla extract and vinegar in a saucepan over medium high heat and cook, stirring occasionally, until the mixture is light amber and a candy thermometer registers 320 degrees, about 10 minutes. Remove from the heat and stir in the 4 cups chips, then immediately pour onto the baking sheet and spread in a thin layer. Gently press the reserved 1 cup chips on tops of the toffee and let cool.
Put about ¾ of the chocolate in a microwave safe glass bowl and microwave 30 seconds. Stir, then continue to microwave in 30-second intervals, stirring between each, until melted. Add the remaining chocolate and stir until smooth, about 4 minutes. Drizzle the chocolate over the toffee. Set aside to harden, about 20 minutes. Break the toffee into pieces.