Combine all of the ingredients in your food processor and process until mixture is completely smooth. This might take several minutes, kind of like when you make nut butter. Be patient!
Transfer the fudge mixture to a small rectangular baking dish that's been lined with parchment paper and use a spatula to spread it in an even layer. Sprinkle tops with fleur de sel or cacao nibs, if using.
Refrigerate until solid, then remove the fudge from the baking dish and cut into squares.
Store the fudge in an airtight container in your refrigerator or freezer. If you freeze it, let it sit at room temperature for a few minutes before serving.