I love love love Italian Meringue Buttercream, but as it sits at room temp with the fondant on, I end up getting buldges. I think I need to create a dam of filling along the edges when I fill my cakes, but how do I thicken IMBC?! Or do I need to use a powdered sugar base buttercream and make it really stiff for that outside dam of filling!? HELP! Also, does anyone have a good ratio of butter/shortening to powdered sugar to get the right stiffness for the dam if I need to use powdered sugar based buttercream?!