How to I prevent my buttercream icing from bulging out between the layers after it has sat for awhile?? I made my daughter-in-law a round 3 layer cake from my smallest round pans, so it was small but tall. By the time we arrived at party location, the cake had humps between each layer and some of the icing was sliding off the side of the cake. Help!!

It needs to settle, preferrably overnight. Use a thicker dam and don't make it right to the edge. That should help!

I typically finish my cakes the day before they are due and placed in the fridge. This gives the cake time to set up. Not sure if you frosted your cake and then traveled with it and had limited or no time in the fridge after icing it?

Was the filling different from the outside frosting? What type of buttercream did you use? I agree with Cakesrock that if you filling your cake depending on the filling you need to build a dam around the edge of each cake layer as a barrier to bulging. I would further add that after placing each layer on top of the frosted layer I gently press the cake down to allow the cake to settle and can correct any bulges at this time.