Gelatin-dairy product combinations: with citrus flavors, use milk to keep acid down; with noncitrus flavors, use milk (sets fastest) or yogurt (low, low fat) or sour cream (richer but treacherous). If you combine citrus flavors and yogurt or sour cream, be sure to add additional baking soda as specified in the basic recipe.
Prewarm thermometer by placing in saucepan of cold water and then bringing to a boil. Grease, and if necessary, line a 5 x 10-inch pan.
Fill sink half-inch full of HOT water (so gelatin does not set up prematurely).
In a 3-quart (unbuttered) saucepan (use a 6-qt pan if using citrus gelatin or yogurt, sour cream, or buttermilk), combine all ingredients EXCEPT butter, flavoring, and optional ingredients. Dissolve sugar, stirring constantly with wooden spoon over low heat until sugar is dissolved. Gritty sounds will cease, spoon will glide smoothly over bottom of pan. However, since you will see granules on the spoon, test by drawing finger - watch out, it may be very hot - through the spoon and feel to make sure it is not sugary. Increase
Boil after washing down any crystals that may have formed with pastry brush dipped in hot water from thermometer bath, using as little water as possible. Introduce prewarmed thermometer. Reduce heat while retaining boil. Stir almost constantly, especially around sides. Depending on the dairy product/gelatin combination you use, mixture may start to cook down and change color slightly.
Shock by placing saucepan in sink of hot water.
Stir when lukewarm and "skin" forms on top (110 degrees F). Return thermometer to its hot-water bath to soak clean. Stir fudge thoroughly but not vigorously by hand (gelatin may stick to bottom of pan so use scraping motion to free it), with electric mixer, or in food processor. Pause frequently to allow fudge to react.
Watch for fudge to thicken, lose its sheen, become lighter in color or streaked with lighter shades, give off some heat, suddenly stiffen. If mixing by hand, fudge will "snap" with each stroke; by mixer, mixer waves will become very distinct; by food processor, fudge will flow sluggishly back to center when processor is stopped.
Add any optional ingredients before fudge totally candies.