After failing to make candied citron the first time, I was left with a pot of salty syrup. I usually make toffee with salted butter and sugar, so the obvious solution was to throw some unsalted butter in the pot and make toffee. To make toffee, you combine an equal amount of butter and sugar, and heat it over the stove until it reaches 285 degrees Fahrenheit.
With a candy thermometer, it's almost impossible to fail at making toffee. You put the pot on the stove, start stirring, and watch the temperature. It's worth noting that at 212 degrees (the boiling point of water) the temperature will stay constant until the moisture from the butter has evaporated.
This can take a while. Keep stirring.
You don't want to walk away from the pot. When the temperature starts to rise again, it will go up very quickly. The whole mixture will foam up and the toffee will burn if you don't stir it constantly and pull it off the heat the second it hits 285. So keep stirring.
When the sun has burned out and all life on earth has perished and the universe has come to an end, the toffee will finally be ready. Pour it onto a foil-lined baking sheet and let it cool, and you will have toffee! You can even drop some chocolate chips on top and spread them around as they melt. When the toffee is hard you can break it into pieces. Of course, all this assumes you didn't get bored and wander away before the toffee hit 285...