Line two large sheet pans with parchment paper or silpat mats.
Add the egg whites and granulated sugar to a heatproof bowl set over a pot of simmering water. Whisk the egg whites and sugar together until the sugar has fully dissolved-- about 2 minutes.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment and beat on medium-high speed for 4-5 minutes, or until the mixture forms stiff peaks.
Add the powdered sugar and almond flour to a fine mesh sieve. Sift them together into the egg whites. Discard any large lumps-- don’t force them through the sieve.
Gently fold the dry ingredients into the egg whites by making a J shape with a rubber spatula. Be careful not to deflate the egg whites at this point.
Once the dry ingredients have been fully incorporated, smush the batter against the sides of the bowl and fold it back together. Repeat this process a few times before checking if the meringue is at the Figure 8 stage.
If you can draw a figure 8 with the meringue without the stream breaking, it’s ready to pipe. If the batter breaks or falls off in clumps, smush and fold a few more times before retesting the figure 8.
When the meringue flows smoothly, transfer it to a piping bag fitted with a round tip (I like a Wilton #10). Pipe 1.5” circles on the sheet pan. Once you’ve finished the first tray, hold it 6-8” off the counter and drop the tray to release any air bubbles. Repeat 4-5 times. Repeat the piping and dropping process with the second tray.
Set the macarons aside to rest until they form a skin-- about 25-30 minutes. You’ll know they’re ready to bake when they don’t stick your finger if you touch them lightly.
While the macarons rest, preheat your oven to 325F.
Bake the macarons one tray at a time for 14 minutes, turning the trays halfway through baking.
Remove the macarons from the oven and allow them to cool to room temperature on the tray.
For the Filling
Add the butter to the bowl of a stand mixer fitted with the paddle attachment. Beat on medium high speed until the butter becomes light and fluffy-- about 2 minutes.
With the mixer on low, slowly add the powdered sugar, followed by the nutella and vanilla. Turn the mixer to high speed and beat the buttercream until it becomes smooth and airy-- about 2-3 minutes. Make sure to scrape down the sides of the bowl a few times so you don’t end up with any chunks of butter.
Transfer the mixture to a piping bag and set aside until ready to use.
Assemble and Age
Place the macaron shells in similar sized pairs. Pipe a small dollop of buttercream onto one of the shells. Place another shell on top and gently press them together until the filling reaches the edges. Repeat with the remaining macarons.
Transfer the filled macarons to an airtight container and place them in the fridge to age overnight.