- 1¼ cups (2½ sticks) unsalted butter, plus more for pan
- ½ cup (packed) golden brown sugar
- 1 tablespoon mild-flavored (light) molasses
- ¼ teaspoon ground allspice
- 2 cups coarsely chopped toasted mixed nuts (such as cashews, almonds, and pistachios), divided
- 5 ounces bittersweet (not unsweetened) or semisweet chocolate, coarsely chopped
Butter a small rimmed baking sheet. Melt 1¼ cups butter in a heavy medium saucepan over low heat. Add next 6 ingredients; stir until sugars dissolve. Attach a clip-on candy thermometer to pan. Increase heat to medium; boil until thermometer registers 290°, stirring slowly but constantly and scraping bottom of pan with a wooden spatula, 20–25 minutes. Remove pan from heat. Mix in 1½ cups nuts. Immediately pour candy onto sheet. Spread toffee to ¼" thickness. Immediately sprinkle chocolate atop toffee. Let stand 1 minute. Using back of spoon, spread chocolate over toffee. Sprinkle with ½ cup nuts. Chill 1 hour. Break toffee into pieces.
Do Ahead: Toffee can be made 2 weeks ahead. Chill in an airtight container. Let stand at room temperature at least 30 minutes and up to 1 hour before serving.