Butter small rimmed baking sheet.
Melt butter in heavy medium saucepan over low heat.
Add the sugar, brown sugar, water, molasses, salt and allspice. Stir until sugars dissolve.
Attach the thermometer to pan. Increase heat to medium; boil until thermometer registers 290, stirring slowly but constantly and scraping bottom of pan with wooden spatula, about 20 minutes.
Remove pan from heat. Mix in 1 1/2 cups nuts. Immediately pour candy onto sheet. Spread toffee to 1/4-inch thickness.
Immediately sprinkle chocolate atop toffee. Let stand 1 minute. Using back of spoon, spread chocolate over toffee.
prinkle with 1/2 cup nuts. Chill 1 hour. Break toffee into pieces.
Can be made 2 weeks ahead. Chill in airtight container.
Let stand at room temperature at least 30 minutes and up to 1 hour before serving.
