Line and grease an 8 x 8-inch baking pan with tin foil and set aside.
In a deep pot or saucepan, add your butter and melt it over low heat. Once it has melted, add the sweetener, heavy cream, and vanilla extract, if using it, and bring it to a simmer. Once it begins to simmer, stir it regularly until it begins to bubble. Use a candy thermometer to check the temperature and once it hits 145C/290F, remove it from the heat.
Transfer the candy into the lined pan and refrigerate until firm. Once firm, break apart into pieces.
* I recommend allulose as it melts just as sugar does. Monk fruit sweetener and erythritol can also be used, but it may turn out a little grainy. TO STORE: Candy should be stored in the refrigerator, in a sealed container. It will keep fresh for up to 1 month.TO FREEZE: Place leftover candy in a ziplock bag and store it in the freezer for up to 6 months.