Line with parchment or wax paper, letting edges of paper hang over sides of pan.
In top of a double boiler or a metal bowl set over (not resting in) simmering water, combine all ingredients except nuts. Mix just until melted and well combined. (Alternatively, use a microwave on low power to melt ingredients, stopping every 10 to 20 seconds to mix well.) The mixture should be heated as little as possible. Mix in nuts.
Scrape mixture into prepared pan. Freeze until set, about 1 hour or so, lift fudge on paper out of pan and use a large knife to cut into squares. Store in airtight container in freezer.
