Step 1

Line a half sheet pan or 9X13-inch pan (for thicker toffee) with parchment paper - no need to butter or grease the parchment. Sprinkle chopped nuts in an even layer over the parchment. Set aside. (I like nuts on the bottom of the toffee, but you can skip this step and sprinkle nuts on top of the toffee after the chocolate melts.)

Step 2

In a heavy-bottom 4-quart saucepan on medium-low heat, add the butter, sugar and salt. Cook, stirring, until the butter is melted. Clip a candy thermometer to the side of the pot, if using.

Step 3

Increase heat to medium, and bring the mixture to a steady boil. Stir only occasionally (not constantly) and avoid scraping down the sides of the pan. Stirring too quickly or too often can cause the toffee to separate. Moderate the heat as needed - turn it down if the toffee is boiling or cooking too fast so it doesn't burn.

Step 4

Cook until the toffee registers 285-290 degrees on an instant-read or candy thermometer and is deep amber brown in color, about 20-25 minutes.

Step 5

Immediately remove the pan from the heat and pour the toffee evenly over the nuts on the prepared baking sheet - don't scrape the sides of the pan. You can spread the toffee with an offset spatula; I pour it back and forth in ribbons over the nuts and let it flow together.

Step 6

Sprinkle chocolate chips on top. Let them soften and melt for 2-3 minutes and then spread the chocolate into an even layer over the toffee. If using nuts on top, sprinkle them over the melted chocolate before it hardens.

Step 7

Chill or let toffee sit at room temperature until set. Break into pieces. Store at room temperature in an airtight container (it will keep for several days at room temp).