Tips for Perfect Italian Buttercream:

Preparation

1. Use soft butter. The temperature of the butter is very important to let all the ingredients blend beautifully. Leave the butter at room temperature ahead to make it soft. 
2. Use very clean tools and ingredients with no oil, liquids, or chemicals attached. They prevent egg whites from whipping well and also could cause sugar crystallizations in the hot sugar syrup. Separate egg whites and egg yolks carefully since yolks contain natural oil. 

Hot Sugar Syrup

1. Ensure the sugar is completely wet before you heat the pot. It's essential to prevent the sugar from crystallizing while heating it. Wash it off with a pastry brush when you see any sugar attached to the sides. (You can also cover a pot with a lid while heating it for the same effect instead of applying the water.)
2. Don't shake a pot or touch the syrup in the beginning. It could cause sugar crystallization when the sugar cools down or touches the air too much. We'll need a little patience here!
3. The temperature of the sugar syrup: Heat it to 244 - 246°F | 118 - 119°C to make a stable, stiff meringue. The lower the temperature is, the looser your meringue gets. And the higher the temperature is, the stiffer your Italian meringue gets. Avoid cooking it more than the temperature to avoid grainy meringue. 

Italian Meringue

1. Make a very fluffy meringue by the time the syrup reaches the temperature (244 - 246°F | 118 - 119°C). The timings are very important. The Italian meringue gets loose when the hot sugar syrup is added to the loose meringue. Also, do not over-whip the meringue until it gets separated. Pay close attention to both at the same time.
2. Add the sugar syrup while it's still very hot. It starts thickening and getting stuck to the pot as it cools down.
3. Add the sugar syrup to the meringue slowly, letting it slide down from the side of a bowl. That way, it gets picked up by a whisk more easily. When you add syrup on top of a whisk, for example, it can scatter to the side of the bowl. Or if you add a lot of it at once, it can sink to the bottom of your bowl. 
4. Continue whipping at high speed initially to make the very fluffy Italian meringue. At this point, the bowl feels very hot. 
5. Stop whipping it when it cools down to room temperature. It'll slowly lose the volume if you continue whipping after it's cooled down.

Buttercream

1. Add soft butter immediately and start mixing it while the condition of the meringue is the best.
2. How much you whip is up to you: The buttercream is fluffy without whipping a lot already thanks to the meringue, but whip more if you want a very airy buttercream. 
3. Clean the bowl, whisk, and a spatula at least a few times on the way to make an even buttercream.
4. Once chilled, it gets firm in the fridge, just like any other buttercream. Serve it right after it's made, or leave it at room temperature for some time after chilling it in the fridge when you want to enjoy the fluffy texture.

How to store it: