The line between softened and melted butter is thin. Turn away from the microwave for a second to chase a wily toddler, and the butter's gone. You can use it in frosting -- with a few caveats. If you're looking for ultra-smooth French or Italian buttercream, forget it, and start over. Melted butter works well, though, for a simple glaze or powdered sugar frosting.
A Thin Glaze
Use melted butter if you're making a glaze for donuts or cinnamon rolls because you want the frosting to be on the thin side anyway. It's a bit tougher if you're looking for a thick buttercream. Melted butter doesn't blend with powdered sugar in the same manner that softened butter does to form a creamy emulsion. Even if you add more powdered sugar, you may end up with a frosting that's greasy instead of fluffy.
The simplest solution is to use the melted butter for something else, like greasing a pan or adding to vegetables, and starting fresh. In a pinch, though, try tossing the melted butter in the freezer for 10 to 20 minutes, just to firm it up a bit. It won't emulsify quite as well as if it had never been melted, but you can use it in cupcakes and kids' cookies. You can also use a combination of melted butter, along with cold butter, cut into small pieces. Combine the melted butter with the powdered sugar first to make a thin glaze. Add the cold butter and beat on medium speed until the frosting is thick and creamy.
Butter has a melting point between 88 and 98 degrees Fahrenheit, while shortening will stay solid to temperatures above 105 F. This is why professional cake decorators often add shortening to frosting when they're serving a cake on a hot day. You can substitute shortening for all of the butter or add some shortening along with the butter. The shortening will help firm up the frosting, while the butter gives it flavor.
Grab an Ice Pack
You've already added the butter to the powdered sugar in your mixing bowl, and you've got a goopy mess. Now what? Pull an ice pack out of the freezer or toss some ice cubes in a zip-top plastic bag. Hold the ice pack against the side of the mixing bowl to chill it -- and the contents inside. Once the butter's a bit firmer, you can try mixing it again. Or, put the mixing bowl in the refrigerator for 20 to 30 minutes to chill and then proceed.