There are plenty of easy recipes for buttercream on the internet but most "easy" recipes call for butter and powdered sugar. This is a fine way to go if you like extremely sweet frosting but if you want to take it up a notch, do yourself a favor and try the Swiss or Italian method of making buttercream instead.
Both methods use egg whites and regular sugar as well as butter to create a silky smooth and not-so-sweet frosting. The main difference between the two is for Italian buttercream, you heat the sugar up to a very high temperature on the stove and then pour the melted hot sugar into your mixer as the egg whites are being whisked. Swiss buttercream, you heat the egg whites and sugar together and then whip them. Both frostings taste extremely similar, I just feel more comfortable not trying to pour scalding liquid sugar into a Kitchenaid as it's mixing.
This recipe makes enough frosting to fill and frost an 8" cake or frost 24-36 cupcakes.
Add the sugar, egg whites, lemon juice and salt to to the bowl of a stand mixer. Whisk by hand until combined. Place bowl over a saucepan filled 2-3" high with simmering water to make a bain marie. Do not let the bowl touch the water!
Whisk constantly while the mixture is heating (so you don't end up with scrambled eggs.) When I am making this recipe for family, I am comfortable heating the mixture until it's very hot to the touch. If I am making this recipe for friends or strangers, I heat the mixture up to 160 degrees using an instant read thermometer. This ensures any bacteria from the eggs is properly killed off.
When the mixture is heated and the sugar is completely dissolved, remove bowl from heat and place directly into the stand mixer fitted with the whisk attachment. Whisk at high speed to make a meringue and continue mixing until it has cooled (10-15 minutes.)
Add the softened butter, 2 tablespoons at a time, to the mixture while the mixer is running. Then add the vanilla. At this point, your mixture may turn soupy or curdle - this is normal. Just keep mixing until it comes together. If your mixture is too warm, place in the fridge for 5-10 minutes and mix again.
Use immediately or store in an airtight container in the fridge for up to a week or in the freezer for up to a month. To bring to room temperature, place on counter and give it time. Do not microwave. Mix again before using to fluff.
***For chocolate buttercream, add 1/3-1/2 cup room temperature ganache after you add the butter.