Dissolve the sugar completely over low heat before bringing the mixture to a simmer. Using superfine sugar, also sold as "baker's sugar," makes this easier. To check whether the sugar has dissolved, scrape the pan bottom with a heatproof spatula, pull the spatula up, let the syrup on it cool for a few seconds, then rub a drop between your fingers. If you can feel grains of sugar, it hasn't dissolved yet.
Prevent sugar crystals from forming on the sides of the pan by brushing down the sides with a wet pastry brush a few times.
Let the mixture cool to lukewarm (exactly 110°) before beating it; otherwise, the fudge may stiffen and become grainy. Pouring it into a large, shallow bowl helps it cool faster, but don't stir it too early.
Beat the fudge well once it has cooled to 110°. Chocolate fudge thickens more than maple fudge at this stage, but both dull slightly and take on a lighter color after beating; that's when they're ready to pour into the pan.