This Simple Trick Fixes Runny Frosting
There are many reasons the frosting recipe you thought was perfect didn't work out. You might have left the butter sitting out for too long, and it's become too soft to work with as a result. You could have added just a bit too much liquid (milk, syrup, or food coloring) that would have sent the balance of solid and cream over the edge. There is also the possibility that your icing might have not stayed in the mixer for long enough (via Bon Appetit).
Pastry chefs offer a variety of fixes for icing which might have crossed over into the runny side, which include giving the icing a rest and having it chill in the fridge for a bit. Another popular way to thicken frosting if you're in a hurry is to add powdered sugar. But if making your frosting even sweeter isn't an option, there is one ingredient you can add that can fix the texture without compromising on its taste (via Leaf TV).
Cornstarch can save runny icing without making it too sweet
Leaf TV suggests that adding a bit of starch — made from cornstarch, tapioca, or arrowroot — is considered one of the best ways to thicken a frosting. The outlet suggests grinding two teaspoons down with a food processor until it is extra fine, then slowly adding half a teaspoon and pulsing until the frosting becomes thick. You may also want to sift the cornstarch as you're blending it into the icing, to avoid the possibility of having a stray lump end up in your otherwise perfect frosting.
Cornstarch is seen as a good way to thicken frosting, because it's used in confectioner's sugar to keep it from clumping. Leaf TV says cornstarch just makes the icing better able to absorb liquids, particularly the one that's made your icing problematic. Cornstarch can also make a buttercream more stable, particularly if a decorated cake is meant to be served on a warmer day.