1. Preheat oven to 350 degrees. Spread almonds evenly on a baking sheet and transfer to oven. Roast until almonds are dark and fragrant, but not burned, about 15 minutes. Remove nuts from oven and let cool completely. Transfer cooled nuts to a large resealable plastic bag. Using a mallet or other heavy object, pound nuts until coarsely ground.

2. Melt butter in an 8-quart pot over medium-low heat; stir in sugar. Continue stirring until toffee reaches 305 degrees on a candy thermometer, about 30 minutes.

Pour toffee onto an 18-by-13-inch rimmed baking sheet, spreading evenly to cover. Let cool until toffee is slightly hardened, but still tacky. Run a knife along the edges of the baking sheet to make toffee easier to remove.

3. Meanwhile, in a bowl set over (but not touching) simmering water, melt chocolate. Remove chocolate from heat and let cool slightly until it reaches about 95 degrees on a candy thermometer. Immediately pour melted chocolate over toffee, spreading evenly to cover. Sprinkle over coarsely ground almonds, pressing down with offset spatula. 4. Transfer to refrigerator until toffee is set, about 1 hour.

5. Invert toffee onto a work surface and remove from pan. Using your hands, break into large pieces. Keep toffee refrigerated until ready to serve.