This Instructable is going to be for a Buttercream that I use frequently. It is basic and almost foolproof. The ratios are simple and easy to convert.

You will need:
Egg whites, preferably room temperature but not necessary.
White sugar
Butter
Vanilla extract
A small chinois or fine mesh strainer.
A scale
An electric mixer or hand mixer with whip attachments

Step 1: Mise En Place

For this recipe you will need:
4oz of egg whites
8oz of sugar
12oz of butter
1 Tablespoon Vanilla x

The ratios for this are very easy to remember. 1:2:3

Egg whites to sugar to butter. Weighed.
I don't believe in using cups as they are very unreliable. So get out your scales! Usually I would make this in large scale batches for working in a commercial kitchen in proportions like 15lbs whites 30lbs sugar 45lbs butter. This makes a rubbermaid sized bin of buttercream and would be made about twice a week. It's a lot.
Today, however, I need a fraction of that. Because of the simplicity of the formula it can be scaled down to ounces without much thought. It translates so easily.

Step 2: Hot Sugar Beware!

Place the sugar and egg whites in a pot and turn your stove on to a medium heat. Stir until the sugar and whites are combined.

In most cases you will want a sugar thermometer. I wanted to make this as accessible as I could and I know not everyone has fancy gadgets or works in the pastry department. You will be using your fingers to test this. As always with hot sugar you need to be careful. You will not be boiling the mixture but it will be higher than body temperature.

You do not need to stir this constantly but I do recommend that you not leave the mixture unattended. I did once and ended up with a delicious caramel buttercream but that outcome was not intended. The point you are waiting for is when the sugar had dissolved. you can test this periodically by letting some of the mixture drip from your spoon and testing the stream coming down for grittiness. This is not exact, which is what comes with trying to make something for everyone, but it will do. Once you have reached that stage remove the pot from the heat. If you have lumps of white that with ok as long as you haven't made sweet scrambled whites. You need the whites and sugar to be liquid.

Step 3: Sweet Meringue

Using your chinois or strainer pour the whites into the bowl of your mixer or the bowl you will be mixing in. The strainer will help to keep lumps out of you finished product.

With the whip attached turn your mixer up to about 6, medium high speed. You want to incorporate a lot of air into this meringue. This portion will take about 10 minutes.
You can check the consistency every few minutes. You are looking for doubled volume and for the whipping marks to stay in the meringue for a minute or so before dissolving. I would say medium peaks.

Step 4: Butter Me Up

At this stage you will add the butter. Turning your mixer down to about 4 you will add the butter in piece by piece incorporating fully after each piece.

When you get about half of the butter in the mixture will look broken. Don't worry--that is only temporary. Continue slowly adding the butter until it is all mixed in. If you have butter on the sides of the bowl stop the mixer and scrape it down with a spatula. Any butter that is not fully mixed in will go rancid and cause your buttercream to taste terrible. Also it will mold. Add in your vanilla extract and continue mixing for another 5 minutes. The frosting should look smooth and cohesive.

Use directly or scoop into a container for later use. I recommend leaving this buttercream out if you plan on using it within two days. Otherwise I suggest refrigeration until you do plan to use it. Remember to remove it from the fridge a few hours before you plan to use it as it needs tempering time and you cannot successfully use it cold. If you need it fast and have forgotten to take it out I suggest beating it with a paddle attachment for about ten minutes. It is not ideal but it is better than nothing.

Enjoy!