Coarsely chop 1 1/2 cups of the walnuts for toffee.
Finely chop 3/4 cup of the walnuts for topping; set aside.
In large heavy saucepan, combine sugar, water, corn syrup and butter.
Heat to boiling over medium heat, stirring until sugar is dissolved.
Cover and cook over medium-low heat 5 minutes.
Cook over medium heat until 300Â°F registers on candy thermometer (hard-crack stage).
Remove from heat, stir in coarsely chopped walnuts and quickly spread in buttered 15x10x1-inch pan.
Let stand until cooled.
Melt chocolate in top of a double boiler over warm, not hot, water.
Spread over cooled toffee and sprinkle with the finely chopped walnuts.
Let stand until chocolate is set, then break into pieces.