Prep the pan. Line an 8x8-inch pan with parchment, leaving overhang on two sides. This makes it easy to lift the fudge out cleanly.
Melt the base. In a medium saucepan over low heat, add butter and heavy cream.
Warm gently, stirring, until the butter is melted and the mixture is steamy but not boiling.
Add chocolate and nut butter. Stir in the sugar-free chocolate chips and nut butter. Keep the heat low and stir constantly until the mixture is completely smooth and glossy.
Sweeten and season. Whisk in the powdered sweetener, vanilla, and salt. Taste carefully.
If you want it sweeter, add 1–2 more tablespoons of powdered sweetener. If adding espresso powder or orange zest, stir it in now.
Optional mix-ins. Fold in chopped nuts or cocoa nibs for texture. Keep the fold gentle so you don’t introduce air pockets.
Pour and smooth. Pour the mixture into the lined pan.
Use a spatula to smooth the top. If you like, sprinkle flaky salt on top for a sweet-salty finish.
Chill until set. Refrigerate for 2–3 hours, or until firm enough to slice. For faster setting, place it in the freezer for about 45–60 minutes.
Slice and serve. Lift the fudge out with the parchment overhang.
Cut into 16–25 squares, depending on how rich you like your portions.