Prepare a macaron template by using a large piping tip or small round cookie cutter of about 1 ½″ in size to trace circles about 2 inches a apart on one sheet of parchment paper. You will place this under another piece of parchment paper when ready to pipe the macaron shells.

In a medium bowl, sift the powdered sugar and almond flour twice.

Next, heat the egg whites and granulated sugar over a double boiler until the sugar has dissolved or until the temperature is about 120 degrees F.

Transfer the egg white mixture to a large bowl or a bowl of a stand mixer fitted with a whisk attachment. Whisk until the meringue reaches soft peaks. At this point, you can add the vanilla paste.

Continue whisking the meringue until stiff peaks form. The best way to test if it is ready is by turning the bowl upside down. If the meringue does not fall or move at all, then it is ready.

Start the macaronage by folding in ⅓ of the dry ingredients. Mix carefully with a silicone spatula by scraping aroung the sides of the bowl, then through the middle of the batter. Do this a few times until it is mostly combined.

Add the remainder of the dry ingredients, folding with the same gentle method. Once the dry ingredients are fully incorporated, begin spreading the batter along the sides of the bowl to deflate it slightly. I find that this mixing method ensures that the shells do not bake up hollow. Continue scraping around the sides of the bowl and through the middle. The mixture is ready when you can draw several figure eights without the batter breaking.

Transfer the mixture to a piping bag with a small round piping tip (I used Ateco 802). Place your macaron template under another piece of parchment paper on a baking sheet and pipe perpendicular to fill in each circle. Carefully remove the template and tap the baking sheet on the on counter a few times in order to release any air bubbles. It also helps to bang on the bottom of the baking sheet with your hand. 

Let the macarons rest for about 30-40 minutes, or until they are dry and no batter comes away when you touch them. Toward the end of the resting time, preheat the oven to 300 degrees F. Bake the macarons for about 8-12 minutes. Allow to cool completely before removing them from the baking sheet.

In a double boiler, heat the egg whites, sugar, and salt, whisking frequently until it reaches 160 degrees F or until the sugar is completely dissolved and the egg whites do not feel grainy.

Transfer the egg white mixture into a bowl of a stand mixer with a whisk attachment. Whisk on high speed until stiff peaks form. Turn the mixer down to low speed, and wait until the meringue cools down to at least 85 degrees F.

While the meringue is cooling down on low speed, reduce the pumpkin puree. Add 200g of pumpkin to a small saucepan and heat over medium heat, stirring frequently. Heat for about 5 minutes, then weigh out 150g in a small bowl. Add the pumpkin pie spice to the reduced pumpkin. Set aside to cool.

Cut the butter into cubes and add to the meringue one piece at a time, allowing each piece to fully incorporate before adding the next. Once all of the butter is added, it will likely look curdled, but keep beating for a few more minutes and it will come together.

Once the buttercream has mostly come together, change the whisk attachment to the paddle and keep mixing on low speed until homogenous and smooth. 

Add the vanilla paste and reduced pumpkin, mixing until fully combined.

Transfer the buttercream to a piping bag fitted with a star tip (I used Ateco 864).

Pair the macaron shells up and pipe the buttercream onto the bottom shell, then place the paired shell on top, pressing down slightly to ensure they stick together.