1 cup water
2 1/4 cups sugar plus 6 Tablespoons
9 egg whites at room temperature
vanilla to taste
Optional: pinch of salt
Whip the butter until it is light and fluffy as possible, or chop it up into small pieces. Add vanilla.
As the butter is whipping place the water and sugar into a pot and cook until the temperature reaches 238 degrees. *OPTIONAL: Using a pastry brush dipped in water occasionally sweep around the edges of the boiling sugar to make sure all of the sugar is incorporated and to keep it from forming crystals on the pot.
Start whipping the egg whites in a clean bowl with clean beaters. Whip until soft peaks form.
Slowly add the hot syrup to the whipped egg whites. Mix the egg whites and syrup until the egg whites are shiny and the peaks are firmer. It will be the consistency of marshmallow.
Now you will need to incorporate the butter into the egg white/sugar mixture.
Slowly add bits of room temperature butter (the butter needs to be soft, you can microwave it slightly to warm it if you need to. You can add it in chunks or pre whip the butter and then add it.) to the egg whites and sugar. It will seem to not mix very well at first (in fact it may look a little curdled for a bit) but it will come together and be very smooth and creamy in the end.
You can store this buttercream in the fridge for a couple of weeks well covered. When you are ready to use it again let it come to room temperature or soften enough so that you can re-whip it.