I need to make enough buttercream frosting to cover about 300 mini cupcakes for a party, I've tried making buttercream frosting before and can't stand the taste, I can taste the cornstarch in it. is there anything I can do so that it doesn't taste awful.

this was basically what I tried yesterday,

1 cup butter

4 cups powder sugar

1T milk

and about 2tsp vanilla

plus some food coloring

I could see that the cornstarch was clumping together in small visible dots all over the place, the consistency was still runny, but I am wondering if I had over beaten it, I was watching a video that said to beat the butter for 5 minutes before adding anything else then beat 2 minutes between each addition of sugar, I had added the milk with the first cup of sugar and the vanilla and food coloring at the end

All I could taste was sweet and cornstarch couldn't taste anything else, though my kids both said it was fine

I was using the walmart brand of powder sugar, just bought some C&H to try

You may try other brands, or you could just purchase powdered sugar without cornstarch, you'll have to research a bit, it is available for purchase.

Make a Swiss meringue buttercream.    The ingredients are butter, sugar, egg whites and whatever flavoring you like. 

Sweetapolita has a great blog post on the process of making it....this is the recipe I swear by

http://sweetapolita.com/2011/04/swiss-meringue-buttercream-demystified/