semi-sweet chocolate chips
Lightly butter a baking sheet(jelly roll pan) and set it aside.
Melt butter in medium saucepan over low heat. Stir in sugar, water and rum.
Once sugar has disolved, add the 1 cup walnut pieces and bring to boil over medium heat until syrup reaches 285 degrees on candy thermometer(about 9 minutes).
Pour toffee onto prepared baking sheet. Using a flat buttered spatula, spread toffee out and allow to cool for 10 minutes.
While toffee cools, put chocolate chips in glass bowl and microwave for 2 minutes and stir until smooth. Spread evenly over toffee and sprinkle with chopped walnuts.
Refrigerate for 30 minutes. Lift off baking sheet with spatula and break into pieces.