Cream butter. Add corn syrup, salt and vanilla; blend. Stir in cocoa. Add milk and powdered sugar alternately, beating until smooth and creamy after each addition. Add enough milk to make a good spreading consistency.
NOTE: When using for smaller cakes or in cake decorating,, refrigerate extra icing in a covered container. Bring to room temperature before using. This keeps well and is useful for quick decorating jobs as beating again is unnecessary. To use for inscriptions, thin slightly with a few drops of water, milk, or coffee.