I'm trying mini cakes for the first time and I'm making a mess trying to us butter cream to seal the crumbs in before I place my fondant on what icing works best for mini cakes?????

Have you thought about using ganache under fondant?

no I have never made that before. How hard is it to make?

It's easy. In a saucepan add 7 ounces of heavy cream and warm until it starts to bubble. Remove from heat and add one 12 ounce bag of semi sweet chocolate chips and stir until melted. I like to add a dash of vanilla. Leave at room temp overnight. Next day frost cakes as you would with butter cream then use the hot knife method to smooth the ganache. Let the ganache set up. If I am in a hurry I will place my cake in the fridge for a couple of hours. So much easier to cover and you will get nice clean edges. Planet cake has a video on You tube showing you how to use ganache under fondant.

Sorry it's late I mean boil not bubble. It's been a long day.

This is why no one likes mini cakes!
It helps if they are still a bit frozen. The type if cake matters, as in if it's a crumby type, you will have more challenges!

It takes a lot of time and patience for mini cakes.
You can stick with bc. Some people melt their bc, and pour it over.
Ganache works well too, you can just pour it over as well.
Some people use marzipan as the base. Brush the cake with hot apricot jam (usually used), you can wrap the sides and put a square on top, means crisper corners.
You probably could do the same techniqe just with fondant too.
poured fondant would be an option too, rather than rolled fondant.
xx