Though toffee is harder than taffy, it is still chewy and sticky—which is why a friend calls it “the dentist’s special.”
Line a large baking sheet with a silicone liner or well-buttered heavy-duty foil, leaving a 2-inch overhang of foil at the two short ends of the pan.
Combine in a large heavy saucepan:
Stir over medium heat until the sugar is dissolved. Wash down the sides of the pan with a pastry brush dipped in warm water. Increase the heat to medium-high, bring to a boil, and boil for 3 minutes. Cook, stirring frequently, to about 280°F, the soft-crack stage. The syrup will be light-colored and thick.
Remove from the heat and stir in:
Pour the candy onto the prepared pan. Do not scrape the bottom of the pan. Let cool for 3 minutes. Sprinkle the hot toffee with:
Let stand for 1 to 2 minutes, then spread the chocolate evenly across the toffee with a small offset spatula. Sprinkle on top:
Refrigerate the toffee for 20 minutes to set the chocolate. Break the toffee into pieces. Store between layers of wax or parchment paper in an airtight container at room temperature for up to 1 month.