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.
Notes
* I recommend allulose as it melts just as sugar does. You can also use monk fruit sweetener, but it may turn out a little grainy. To store: Store leftover candy in an airtight container in the refrigerator for up to one month. If you prefer softer candy, store it at room temperature. To freeze: Place leftovers in a freezer bag and freeze for up to six months. I actually really like sucking on them straight from the freezer (strange but good).