I used a "French" Meringue buttercream recipe which I found out was not a true French Meringue but that's beside the point. Right now I have a buttercream that is so thick and buttery there is no way I can use it on my cake for tomorrow. Is there something I can do to fix this? Can I whip up more egg whites and sugar and fold/mix it into the batch? I'll post the recipe measurements below that I used.

10 egg whites
2 cups sugar
7 sticks of butter (it called for 10!!! but I knew that was too much)

you just whip the whites til foamy, slowly add the sugar, whip to stiff glossy peaks. Gradually add the butter and whip it until it comes together. I had even added about a cup of lemon curd and it still is thick and way too buttery!

I saw that recipe a while back when I was looking up what a French meringue b/c was. The comments at the end of the recipe were exactly like what you've experienced. I'm not really sure how you can it because it's a french meringue (egg whites and sugar) to which they've added butter.

I know it's an expensive mistake. but to be honest I would throw it out and start again.

I tried to find you some info on line but was unsuccessful. I did find this link which looks useful to anyone who wants more info on buttercreams and some really good recipes.

http://www.pastrysampler.com/Questions_and_Answers/buttercream.htm

More eggs and sugar!! do til they are glossy then add to the org.
mine is 16 eggs, 4.5 sugar 10 butter

Thank you both so much! I'll take a look at the link but I think I will add more whites and sugar to see if that helps.

Sounds like smbc w/o cooking the eggs & sugar. The recipe I use is 4 - 8 - 12 oz. Eggs sugar butter. I usually cut down on the butter. To increase just keep using that ratio. Whatever the first number is, double then triple for the remaining ingredients. Flavor as desired.

French Meringue Buttercream is based on the French "meringue Suisse or meringue ordinary". This is just egg whites and sugar whisked until stiff with a light texture, to which butter is added. This is a light Buttercream.

Easy Recipe
3 egg whites
16 oz powdered sugar
1 ½ lb unsalted butter
Beat egg whites until stiff. Set aside.
Beat butter and powdered sugar until smooth.
Beat in egg whites in 2 separate beatings (½ at a time).
Beat well after each addition.
[Add vanilla to flavour].
Notes:
When the butter is first added, the meringue will break down.

Raw eggs? How does this combat bacteria\\salmonilla? Does the sugar neutralize it? Is it lighter thn smbc or imbc?

The recipe is a traditional recipe that has been around forever. Raw eggs didn't present a problem.

As the US has a problem with salmonella, I wouldn't recommend this as a useable recipe. Unless, of course, you could guarantee the source of the eggs. Try pasteurized eggs, perhaps ?