In a large saucepan, bring cream to a boil. Add chocolate and stir until melted. Remove from heat; add sugar substitute, butter, and vanilla extract; mix well. Let cool until mixture is stiff, about 2 hours.
Sift cocoa powder onto a piece of waxed paper. Use a small melon ball scoop to form chocolate balls (or use 2 spoons.) Roll in cocoa powder. Place truffles in foil candy holders if desired.
Total carbohydrates per truffle: 2 net carbs; 1 gram protein; 5 grams fat; 50 calories.