Reduce heat to medium-low. Continue cooking, stirring occasionally, until candy thermometer reaches 300°F or small amount of mixture dropped into cold water forms hard brittle strands (25 to 40 minutes). Stir in 1 cup nuts.
Pour mixture into prepared pan; spread to desired thickness. Sprinkle chocolate chips over hot candy; let stand 5 minutes. Spread melted chocolate evenly over candy. Sprinkle with nuts. Cool completely (at least 3 hours). Break into pieces. Store in airtight container in a cool dry place up to three weeks.
If your toffee separates during cooking, carefully and slowly add about 1/2 cup hot water, 1 tablespoon at a time, and stir constantly.
If you prefer a thinner toffee, use two buttered (15x10x1-inch) pans or cookie sheets and pour half of toffee mixture on each; spread to desired thickness.