Line an 8x8 inch baking dish with parchment paper, leaving an overhang on the sides for easy removal. This prevents the fudge from sticking to the dish.

In a double boiler or a microwave-safe bowl, melt the sugar-free chocolate chips (or unsweetened chocolate). If using a microwave, heat in 30-second intervals, stirring well between each interval, until the chocolate is completely melted and smooth. Be careful not to overheat or burn the chocolate.

In a medium mixing bowl, combine the melted chocolate, heavy cream, and butter. Stir until the butter is melted and the mixture is smooth and creamy.

Add the sugar-free sweetener (erythritol or allulose) and vanilla extract to the chocolate mixture. Stir until the sweetener is fully dissolved and everything is well combined. If using, fold in any chopped nuts, sugar-free candies, or other desired mix-ins.

Chill the fudge in the refrigerator for at least 2-3 hours, or until it is completely firm.

Once the fudge is firm, remove it from the dish using the parchment paper overhang. Cut it into squares and serve.