[make vanilla and chocolate fudge in one batch by adding the vanilla first, beating it and pouring half the mixture out, then quickly adding a few squares of dark chocolate before you pour the rest]
- it will be sufficiently boiled when the bubbles subside such that the level of liquid retreats back to where you started, and, more precisely, when it is at the 'soft ball' stage, i.e. when a spoonful of the hot mixture dropped into a glass of cold water forms a lump of soft caramel. This caramel is very tasty, so you may wish to try more than once to be extra sure.
- it will be sufficiently beaten when the colour changes quite suddenly - it will become opaque and have a grainier texture. At the very point when you start wondering whether you have beaten it so much that you'll have difficulty pouring it out before it sets, then it's time to stop. Your arm will be tired by then anyhow.
[is this enough fudge?]
And another tip: always use a round plate or dish in which to leave the fudge to set. This will conveniently leave lots of non-square bits round the edge which pride would prevent you from serving to visitors.