Okay, So I really like the taste and texture of this buttercream I've been making. It has 1 cup shortning, 1 cup butter, 2lbs. powder sugar, some water, salt, and flavoring. The only problem is it seems a little too soft-some of my borders started to "melt" off one of my cakes. What is the best way to fix this? Should I just add more powder sugar? I was reading one Wilton recipe for High Humidity Frosting that added 2 T of cornstarch. I do live in Texas so humidity is def a factor...

reduce the fat or add more sugar. My recipe only uses 1-1/3 cups of fat to 2 lbs of p.sugar.