In a large, microwave-safe bowl, combine all ingredients (except for the nuts.) Microwave, in 30 second bursts, and stir. Do this just until the chocolate is melted. Don’t over-heat chocolate or the fudge will get grainy. Stir in nuts, if using.
Pour into prepared pan. Smooth top (use fingers if necessary.)
Refrigerate until completely set.
Cut into squares. Place in an airtight container and keep in a cool, dry place. Sprinkle with a little powdered sugar just prior to serving.