2 sticks unsalted butter, plus more for the baking sheet
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 butter the foil. Set 1 cup potato chips aside; break the remaining 4 cups chips in half if they're large.
Combine the butter, sugar, vanilla 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 F, 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 top of the toffee and let cool.
Put about three-quarters 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.