Line a 9x5-inch loaf pan with parchment paper, leaving overhang on the sides for easy removal.
In a food processor, combine the avocado flesh, melted coconut oil, cocoa powder, maple syrup, vanilla extract, peppermint extract, and sea salt. Process until completely smooth, scraping down the sides as needed. Taste and adjust sweetness or mintiness.
Pour the mixture into the prepared loaf pan and spread evenly with a spatula. Sprinkle chopped dark chocolate over the top if using.
Freeze for at least 2 hours, or until firm. For best results, freeze overnight.
Lift the fudge out of the pan using the parchment overhang. Cut into 12 squares. Serve straight from the freezer. Store in an airtight container in the freezer for up to 3 months.