How do I make the Buttercream shiny on a cake? Is it a special buttercream recipe? I would like to to look like the ones from the supermarket bakery. they get it sooo shiny

The only way I know to make buttercream "shiny" is to use pearl spray. I kinda doubt grocery stores are using pearl spray though. It's possible the shiny effect you're seeing is condensation from taking the cake out of the freezer.

I know others have made fondant shiny by using a steamer on the cake once it's done.