Combine the chocolate chips, nut butter, and syrup in a double boiler or in a bowl perched over a saucepan in which water is gently boiling. Cook over very low heat until the chocolate is melted, stirring frequently once it starts melting. You can also do this in a the microwave; combine the three ingredients in a microwave-safe bowl; Start with 45 second; stir, and add a few seconds at a time. Either way, the mixture should be smooth and thick.
Remove from the heat and stir in the seeds and walnuts, and optional cacao nibs.
Scrape the mixture into an 8- by 8-inch or 9- by 9-inch baking pan. Smooth it in evenly, patting in with the back of a small spatula.
Sprinkle the top with the optional salt (use the salt sparingly; it’s just for a nice accent, not to make the fudge salty!
Score into 24 or 36 little squares with a butter knife. Refrigerate the fudge uncovered for at least an hour. If you’re in a hurry, place in the freezer for about 30 minutes. Store any not eaten immediately in the refrigerator, covered.