Butter an 8-inch-square baking pan.
Line with parchment or wax paper, letting edges of paper hang over sides of pan.
In top of a double boiler, 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, if using.
Scrape mixture into prepared pan.
Refrigerate until set; about 4 hours or overnight.
Lift fudge, on the paper, out of pan and use a large knife to cut into squares.
