We followed all the same steps above for the Italian meringue used to make the macarons. Here they are again for convenience.Just like before, add the granulated sugar and water to a small sauce pan on low heat. We want to heat the sugar to 240-245°F. It will be syrup-like, but not caramelized. While it is heating, whip up the aquafaba. In the bowl of a stand mixer, or a large mixing bowl, add 50ml of aquafaba and ¼ tsp of cream of tartar.
Whisk on high speed until stiff peaks form. It will go from bubbly, to foamy, to a glossy foam.
Once the sugar reaches 240°, turn off the heat. Slowly and carefully stream in the sugar down the side of the mixing bowl while the aquafaba continues to whisk on high speed. Try to avoid pouring over the whisk because it will send more of the sugar flying to the sides of the bowl rather than into the aquafaba. The sugar will also be extremely hot and any splashes could burn you. It's okay for there to be some sugar that remains in the pot that can't be poured out. Don't worry about the specks and drops of sugar on the sides of the mixing bowl either.
Continue to whisk on high speed until the bowl of the mixer is cool to the touch. It will be extremely hot when you first pour in the sugar. Our bowl cooled down between 2-3 minutes. You will see the aquafaba become really glossy and start to thicken as it gets mixed. This is the Italian meringue.
Next, using room temperature butter, add about a tablespoon at a time to the Italian meringue. We just estimated it--sometimes we added more and sometimes we added less. Allow each addition of butter to fully incorporate into the meringue.
Once all the butter is added, you should have a smooth and fluffy mixture. This is the Italian meringue buttercream. You can add in the flavorings or colors you would like at this point.
You can add purple/violet gel food coloring to the buttercream to offset the yellow tint from the butter. This will make it appear a bit whiter. Use the smalled bit of coloring at a time. You can add it in with the tip of a toothpick or skewer.
We added ½ tablespoon of instant coffee powder to our buttercream to fill some of our macarons.
Here is what it looked like once it finished mixing.
We placed the buttercream into a piping bag with a star-shaped tip and then piped the buttercream onto the bottom of a macaron shell. Finish the sandwich with a second shell on top.