UPDATE: Adding some crushed peanuts to the mixture gives great texture to this fudge. My husband loves that. I'll make it again and figure out just how much to use - but probably about 1/3 cup crushed peanuts. :)
If you don't have cocoa butter, I'm guessing replacing it with 60 g (2 squares) unsweetened baker's chocolate should work. Increase the sweetness level to taste. The erythritol will need to be doubled at the very least.