Two Cups Flour

0 Separate eggs whites and place them into a clean bowl of a stand mixer.
14 Add the sugar to the eggs. Create a double boiler by placing the bowl over a pot of shallow simmering water. Do not let the mixing bowl touch the water.
1 Whisk the egg whites and sugar continuously until the temperature on an instant thermometer reads 160℉ (72℃). Carefully dip your finger into the hot sugar and rub the mixture between your fingertips. If you don't feel any grains of sugar, the mixture is ready. (about 6-8 minutes)
2 Remove the mixing bowl from the heat and wipe the bottom of the bowl dry with a towel. Place the bowl on the mixing stand fitted with the whisk attachment and begin whipping on high speed. 
12 Continue to whip until a thick, glossy and marshmallow like meringue with stiff peaks is formed. The bowl should be cool or at least room temperature to the touch. (about 10-15 minutes)
3 Switch to the paddle attachment on the mixer. (You can continue with the whisk attachment if you prefer and switch to the paddle attachment at the end to beat out any air bubbles.)
4 While mixing on medium speed drop in one tablespoon of butter at a time into the whipping meringue. Wait for each piece of butter to mix-in before adding the next. (I have also added half the butter at once, then the second half without any issues.)
5 You may notice the meringue losing its stiffness and becoming softer, that's ok, keep adding butter.
6 Once half of the butter has been mixed in, stop the mixer and scrape the bottom and sides of the bowl to grab any stuck meringue or bits of butter. We want all the butter to emulsify with the meringue.
7 Continue mixing and add in the remaining butter.
8 The meringue may deflate and look runny or split, but keep mixing for another 10-15 minutes after all the butter has been added on medium speed.
9 If after 10 minutes of mixing it hasn't magically transformed into a frosting texture, just place the bowl in the fridge for 10 minutes with the paddle attachment. Then, whip again until silky smooth.
11 Once the buttercream looks like a spreadable frosting add in the vanilla bean paste or vanilla beans and give it a couple spins on low to mix. 
10 If there are a bunch of air bubbles in the frosting, mix on low speed with the paddle attachment until smoother. Or simply take a silicone spatula and beat the frosting left and right in the bowl, by hand to loosen up an air bubbles.