Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy lifting. Lightly grease the parchment.

In a medium heavy-bottomed saucepan, whisk together the sugar free sweetener and cocoa powder until no lumps remain.

Add the butter and heavy cream to the pan. Set over medium-low heat and stir constantly as the butter melts and the mixture loosens.

Bring the mixture to a gentle simmer, stirring often, and cook for 3–5 minutes until it looks glossy and slightly thickened. Do not let it boil hard or scorch.

Remove the pan from the heat. Add the chopped sugar free dark chocolate, instant espresso powder (if using), salt, and vanilla.

Let the mixture sit undisturbed for 1–2 minutes to soften the chocolate, then stir slowly and continuously until all the chocolate is melted and the fudge base is smooth, thick, and shiny.

If using nuts, coconut, or peanut butter, gently fold them into the warm fudge until evenly distributed.

Immediately pour the fudge mixture into the prepared pan and smooth the top with a spatula, tapping the pan lightly on the counter to release air bubbles.

Let the fudge cool at room temperature for about 30 minutes, then transfer to the refrigerator and chill until firm, at least 1 1/2 to 2 hours.

Once set, lift the fudge out of the pan using the parchment overhang. Place on a cutting board and use a sharp knife to cut into 1 1/2-inch squares.

Store the fudge in an airtight container in the refrigerator for the best texture. Let sit at room temperature for 5–10 minutes before serving for a softer, slow-melting bite.