Mix sugar with cream, and stir until dissolved. Add cream of tartar, and boil to 238°F., or until soft ball forms in cold water, stirring carefully to prevent burning. Move thermometer often so fudge will not burn underneath.
Pour onto a lightly oiled marble slab or silicone mat. Allow to cool, and work fudge with a broad spatula or butter paddle until creamy. Cover with a damp cloth for 1/2 hour, add vanilla extract, and work in well with hands.
Place in small, shallow box, previously lined with wax or parchment paper. When cold, cut into squares.
Shape vanilla opera fudge into small balls, using about 1 tablespoon fudge for each ball. Press 1/2 walnut meat firmly into each side, and roll in granulated sugar, either plain or colored.
