Prepare 8x8 pan by greasing it with butter. If you want thicker fudge you can use a smaller pan (it may take a bit longer to set at the end).
Combine ingredients in a small pan on the stove-top and bring to a rolling boil. Using a candy thermometer to check, bring it to 238F (114C).
Alternately, you can use the "soft ball test". Drop a small amount of the mixture into a cup of very cold water. If it blobs together and is pliable and soft when you remove it, it's ready. If it's not yet ready it will dissipate in the water into little bits.
Remove from heat to cool until mixture is between 100F and 110F. For faster cooling, you can place your pot into a cold water bath in the sink.
When it has cooled adequately, beat the mixture until it loses its shine. It should be completely dull with no shine left.
Note! If using a handheld mixer, be careful you don't burn out your motor, as this might take 10+ minutes of beating on high. I recommend giving your mixer a little breather every 4-5 minutes or so, or else you might begin to smell burning metal, ahem. A stand mixer is obviously superior for this, but that dirties another dish (plus, I don't have one, so there's that).
Spread the mixture quickly into the prepared pan. It will begin to set immediately. Cool in the fridge for around half an hour (or until you can't stand it anymore) and then indulge. Slowly, with your eyes closed.
Rinse, lather, repeat.
Notes
*nutritional info calculated for cutting 36 pieces