A two-part recipe. Make the fondant first and set aside, then the fudge layer. FONDANT LAYER: Combine sugar, salt, corn syrup, and boiling water in a 2-quart saucepan. Cover and bring to a rolling boil. Remove lid and place thermometer in pan; cook without stirring to 240 degrees F. Pour out on marble slab. Cool to lukewarm and work with spatula until creamy and white, then knead smooth with hands. Add vanilla and marshmallow. Work into fondant. Let fondant stand, uncovered, until completely cold. For layered Fudge Squares, pat and roll fondant into an 8-inch square between two pieces of waxed paper. Set aside. NOTE: To store fondant, wrap in waxed paper and place in a tightly covered jar. Keep in refrigerator. FUDGE LAYER: In the top of a double boiler, melt milk chocolate or sweet coating over hot, not boiling, water. Heat cream, half and half, and vanilla to scalding. Remove from heat and cool to about 130 degrees F. Do not allow temperature to drop below 125 degrees F. Add warm cream mixture to melted chocolate all at once and beat until smooth and well blended. Remove from hot smooth and well blended. Remove from from hot water and let cool until pliable. Beat with an electric mixer until candy is light and fluffy. Spread half in an 8-inch square pan which has been lined with lightweight foil. Place the fondant square over the fudge; spread the remaining fudge over the fondant. Sprinkle with nuts, and press the nuts into the fudge. Cut into squares when firm, and wrap in waxed paper. Makes about 48 pieces.