Sprinkle walnuts in the bottom of a 9-inch round cake tin.
Combine sugar and butter in a 1 1/2-quart saucepan. Cook and stir over medium
heat until the mixture reaches 290 degrees F on a candy thermometer. Stir the
toffee constantly while melting the brown sugar and butter.
Remove melted toffee from the heat and cool slightly for about 5 minutes.
Pour the mixture over the top of the walnuts in the pan and spread out in
an even layer.
Immediately sprinkle the top with the chocolate chips. Let the chips melt
from the heat of the toffee, then gently spread the melted chocolate over the
top of the hardening candy.
If desired, sprinkle a few more chopped or ground walnuts on top of the
chocolate while it is still soft and warm.
Cool at room temperature until hard, at least 2 hours, then break into pieces
and store in an airtight container.