Sift powdered sugar, cocoa, and salt into a 1 quart microwave safe bowl. Stir in milk and vanilla. Mix well. Place butter on top. Microwave on high, 2 minutes.
Beat with wooden spoon until smooth. Stir in nuts (OPTIONAL).
Spread in 8 X 8 X 2 inch baking pan.
Chill about 1 hour or until firm. Cut into pieces.