So dummy that I am, am a frazzled mess today. I made my sons cake smash and had 2 whoopsies with my buttercream that now I am worried I ruined them.
1) Half the batch I put too much powdered sugar in. I put in 8 cups when I meant 6 and everything else in the recipe was balanced to 6. What will this do to the buttercream?
2) The other half of the batch I got the icing sugar right but then forget the darn vanilla. Bang up night for me! So it is just butter, shortening and icing sugar, no flavoring. What will this do?
You'll be fine. The extra sugar will just make it more stiff than usual, and no vanilla is no big deal.
It's certainly very easy to add vanilla at this point. Just stir it in. And as costumeczar said, the other 1/2 will just be a bit more stiff. If you have trouble piping with it just thin it down by adding any liquid = say about 1 or 2 tablespoon.