Melt chocolate slowly by microwaving on power level 8 in 45 second intervals, stirring frequently, until chocolate is melted.

Place paper cupcake liners in a cupcake pan. Drop a dollop of chocolate into each liner and spread to evenly cover the bottom. Brush chocolate up the edges slightly with a spoon or pastry brush. Place in the freezer to harden.

In a microwave safe glass bowl, heat the cold butter and 3/4 of the erythritol on power 8 for three minutes. You must stir the mix every 20 to 30 seconds to prevent burning! The mixture will still look very watery and will be extremely hot! Add the remaining erythritol, then stir to thicken. Add the chopped walnuts.

Remove the chocolate cups from the freezer. Fill each cup with a half spoon full of toffee mixture. If the mixture begins to separate and harden, that is normal! Just stir it gently and work quickly.

Top each cup with remaining chocolate, reheating if necessary, and cool in the refrigerator for 1 hour.

Remove from cups and sprinkle with sea salt!