Frosting kept melting when trying to frost cake I made a chocolate malt cake for the first time the other day.  I let the cake sit overnight to cool.  I made the frosting while the cake was baking and put it in the fridge for the next day.  The cake recipe had me cook the cake at 325 for 45-50 minutes.  That turned out to be 10-15 minutes too long (when I checked it at 40 minutes it was very overdone).  That's easy to correct for next time.  The frosting is what's stumping me.  The recipe is:


3 sticks (12 ounces) unsalted butter, at room temperature
1 1/2 cups powdered sugar
3/4 cup malted milk powder
1/2 cup whole milk, at room temperature
2 teaspoons vanilla extract
1/4 teaspoon kosher salt


When I added the first three ingredients it was a good consistency, if slightly thick.  I added the rest and it became too thin.  I added another 1 1/2 cups of powdered sugar and it was a lot better.  I put it in the refrigerator to use the next day.  Of course it became very firm.  When I was spreading it onto the cake it kept getting so melty that I couldn't spread it consistently.  I've never seen this happen before.  It was so bad that I actually had to make a ganache to cover the bad frosting job.  Next time I will definitely cut down the butter to two sticks and reduce the milk to maybe 1/4 cup.  What else could have happened to cause this?
There are two things I would consider with the frosting. The first is that either the cake or the kitchen was too warm when you tried to frost the cake. It's happened to me a few times that I set the indoors temperature too warm in the winter and frosting keeps melting for me. Freezing the cake for about 20 minutes before attempting to frost it will usually make it easier to do a nice job too.

Otherwise, when you make a frosting recipe like that one, you want to mix all the ingredients except the liquid (in this case milk) together first, and the add the liquid, a little at a time until the frosting is the right consistency. Never add all the liquid at once. I don't think the butter was the problem, if anything you didn't have enough of it compared to the amount of milk.