Melt chocolate in microwave-safe bowl on low at one min. intervals, stirring until just melted and smooth. Or, using a double boiler, place chocolate in top bowl, stirring frequently until just melted, taking care not to overheat at bottom or sides. Allow melted chocolate to cool enough to touch.
Place the cooled butter in a medium mixing bowl and with an electric mixer (fitted with the paddle) or wooden spoon, beat until light and fluffy. Scrape down the sides.
Beat in the cooled chocolate, cocoa, zest and sugar scraping sides of the bowl with a rubber spatula as needed to incorporate all ingredients. Stir in nuts if desired.
Scrape into a small bowl, serving crock or butter mold and cover tightly; or shape into a long roll in grease-proof paper (plastic, wax or parchment) for storing and slicing as needed.

*This is a place to use the best quality chocolate you can find, preferably one with 70% to 85% cocoa content.
**Roasted and salted pistachios add a welcome textural contrast. Most stores sell them already shelled. You may also use a different toasted nut or no nuts at all.