My preferred

buttercream

After trying a few types of icings, this buttercream (Italian meringue buttercream) has become my favorite to use when icing cakes. I find that it gives me the smoothest results with the sharpest edges, which is exactly what I am looking for.

NOTES:

When making this recipe to ice and fill my 4 layer, 6-inch cakes, I usually make 1 - 1.5x the recipe. If you're unsure of how much you'll need, I would recommend scaling up to be on the safe side. It's better to have a little extra icing than not enough.
This is a plain buttercream, intended to be flavored to your liking. Please refer to notes about flavoring below the recipe.

*I have the scale I use, along with other cake & icing making products, linked here*

My preferred buttercream (Italian meringue buttercream)

*You will need a candy thermometer for this recipe, linked here*

Ingredients:

16 oz. (4 sticks) unsalted butter, room temp
9-14 oz. sugar depending on how sweet you want it
5 egg whites [or about 5 oz. if using cartoned egg whites]
1/8 tsp. cream of tartar

Put sugar into a pot and add enough water to cover the sugar. Stir and place on a stovetop over medium high heat.
While sugar mixture is heating, add egg whites to a clean & dry mixing bowl. Add cream of tartar. With whisk attachment of mixer, whisk until combined.
Continue heating sugar mixture until it reaches 240°F, then remove from heat.
Put mixer on a low/medium speed and slowly pour hot sugar into egg whites while mixer is running.
Once all of the sugar is added, turn mixer speed up to medium/high and continue whipping until mixture is cool to the touch.
Once cool, add room temperature butter to the mixture, approx. 1 tbsp at a time.
Continue whipping until everything comes together and creates a silky icing.
This is the point where I would add any flavorings or colors that I may want.
TIP: For smoother buttercream with fewer bubbles, use the paddle attachment on a low speed for a minute or two after buttercream comes together.

NOTES

Notes on making this buttercream:

If you add the butter while the meringue (hot sugar and egg white mixture) is too warm you may end up with SOUPY icing. If this is the case, refrigerate icing for a few minutes and mix again. It should thicken up and become the right consistency.
If you have made this in advance and stored it in the fridge let it sit on the counter to come to room temperature and re-whip before using. If you don't have the time to wait, you can put all of the buttercream in a mixing bowl, reserving a small amount. Put the small amount into the microwave until melted and add to the cold buttercream. Mix until icing becomes smooth again.
- Your buttercream can become soupy if you melt too much of it, so make sure you're melting small amounts and adding them in slowly.

Notes on flavoring this buttercream:

I prefer my base icing to be sweeter than most recipes, so I use up to 14 oz. of sugar per batch, but I've used anywhere from 9-22 oz. in the past and have not had a problem. If you like your icing less sweet, I would start at 9 oz. Adjust the amount of sugar to your sweetness liking.
To make this buttercream a vanilla buttercream I typically use 1-2 tsp. of vanilla per batch, but I'd recommend tasting the buttercream and adding as much as you see fit.
Melted and cooled chocolate can also be added to buttercream. You can use about 5 oz. per batch. Pour in the cooled chocolate while the mixer is running on a slow speed.
You can also mix in cookie crumbs to flavor the icing. Just grind the cookies in a food processor and mix them into the buttercream. Note that this will make the icing "dryer" and less spreadable the more cookie crumbs you add. If you plan on using the cookie crumb buttercream between the layers, you can use more crumbs in the icing, but if you plan on spreading it on the outside of a cake, I would recommend using a smaller amount so the icing remains spreadable.
Fruit reductions are another great additive to buttercream. If you plan on adding purees or another form of liquid to the buttercream, however, add it in slowly because too much liquid can "break" the buttercream and it will no longer have that smooth texture you want.

Notes on coloring this buttercream:

I prefer to use gel food coloring for this buttercream. My most used food coloring is Wilton Color Right Food Coloring System, but if I know I'm going to need a lot of a standard color I like Wilton Gel Icing Color.
If you want to make black buttercream, please visit my page on creating black buttercream, here.
For deeper colors, let the colored buttercream sit at room temperature overnight or for a day to deepen the color.