Line a small baking pan with foil and spray with cooking spray.
Melt butter over low medium until it starts to turn golden. Add nuts and cook for about five minutes until they are slightly toasted. Add 4 tbsp of the cream and the sweetener. Cook until thickened and golden. This takes about 10 minutes. If it doesn't seem to be thickening you can turn up the heat to medium or medium high but stir constantly. It can burn quickly. Remove immediately from the heat and add the salt and reserved cream. Stir until smooth. Pour onto the foil lined tray and refrigerate until firm to the touch (about half an hour).
Melt the chocolate. I do this in a glass bowl in the microwave stirring every thirty seconds. Add sweetener if needed. Pour half of the chocolate on top of the toffee. Put in the freezer 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 chocolate side down. Pour the rest of the chocolate on top. Refrigerate or freeze until firm.
The filling stays soft like a caramel for the first few hours but hardens like a toffee overnight.
***I used two 3.5 oz Lindt 90% bars and added 1/3 cup sweetener- make sure to use chocolate that you like - you can really taste it in this recipe - if you use very dark or unsweetened chocolate just sweeten it to taste