How hot should my water be? The water here should be quite hot - nearly boiling - for best results. We usually heat water to the highest setting in an electric tea kettle, but you can also heat it in a small saucepan on the stovetop or (carefully) in the microwave. If you don’t have the time or equipment to heat water this way, you can use water straight from the tap on its hottest setting - it won’t keep the frosting soft for quite as long, but it will still work! 

How much frosting does this make? As written, this recipe makes enough for a thin layer of frosting on one large sheet cake, a very thick layer of frosting on an 8-inch square cake, or a generous dollop of frosting on 10-15 cupcakes. 

Fudge frosting will seize and harden as it cools, so work quickly to frost your cake. If the frosting is too thin (this often happens when too much hot water is added) add extra powdered sugar or let it cool on the counter for a few minutes to thicken it up. If frosting is too thick, stir in an extra splash of hot water or zap it in the microwave (using a microwave-safe bowl) for 5-second intervals to loosen it. 

This frosting works best on cupcakes and sheet cakes. As it cools, the frosting will become more difficult to spread, so use it on low-maintenance cake styles like a sheet cake or cupcakes (rather than a layer cake) for best results. Because the consistency changes as it cools, we don't recommend piping this frosting with a pastry bag.

Frosting will last up to 4 days in an airtight container on the counter once it’s applied to your cake or cupcakes. After a few days, the frosting will lose some of its texture and start to release some of its moisture, which can make the cake below a little bit sticky. For best results, frost your cake as soon as you mix the frosting, and eat it within a day or two!

For a dairy free and vegan frosting, use melted coconut oil in place of the melted butter here.