This is especially important if you’re working in a warm kitchen. Check the frosting after 30 minutes to see if it has thickened by sticking a spoon in it and stirring it around. Putting the frosting in the fridge will also help it thicken if the frosting was over-mixed.
You’ll know the frosting has thickened when it sticks to a spoon without dripping when you lift the spoon out of the bowl. It’s best to add the extra powdered sugar in small amounts to make sure you don’t end up adding too much.
Avoid adding more than 1 US tbsp (15 ml) of cornstarch to the buttercream frosting, or the frosting will have a starchy flavor.
Melting the butter too much before pouring it into the frosting will mess up the frosting’s consistency. If your butter has turned into a liquid, it’s too melted. Press down on the stick of butter using a spoon. If the spoon moves through the butter easily, it’s ready to go into the frosting.
If you’re mixing the frosting by hand, try mixing it for at least 3-5 minutes to see if it starts blending better.
Stir the milk into the frosting for at least 10 seconds before deciding whether or not to add more.
If your frosting curdled, it’s likely because one or more of the ingredients weren’t at room temperature before you combined them.
Mix the warmed frosting into the regular mixture on a medium speed if you’re using an electric mixer. If one of your ingredients is significantly colder than the others, they’re not going to mix well together, creating a cottage cheese-like product instead of smooth buttercream frosting.
