Frosting kept melting when trying to frost cake

I made a chocolate malt cake for the first time the other day. The 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?