Melt butter in a pan. Add sugar, corn syrup and salt. Heat (using a candy thermometer) until golden brown and the temperature reaches 300 degrees F. Pour the toffee mixture over the nuts, spreading it thin with a metal spatula. Immediately sprinkle chocolate on top.
Add sweetener if needed. Pour half of the chocolate on top of the toffee. Put in the freezer for a few minutes until the chocolate is solid. Flip over the toffee bark and peel off the foil. Put the foil back on the tray and put the toffee on the chocolate side down.
Directions. Quickly pour into prepared pan. Let stand at room temperature until cool, about 1 hour. In a microwave, melt chocolate chips;stir until smooth. Spread over toffee. Sprinkle with pecans. Let stand for 1 hour Break into bite-size pieces. Store in an airtight container at room temperature.
Recipe makes 2 1/4 pounds of toffee. Toasting nuts before adding to a recipe intensifies their flavor. Toast nuts in an ungreased skillet over medium heat, stirring frequently, 3-4 minutes or until golden brown. To oven-toast, place in baking pan.