Line two large sheet pans with parchment paper or silpat mats.
Heat a small pot of simmering water over medium-low heat. Add the egg whites and granulated sugar to a heatproof bowl and set it over the simmering water. Whisk constantly until the sugar has fully dissolved-- about 2 minutes.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment-- be careful not to get any water in the bowl!
Beat on medium-high speed (Kitchenaid #8) until stiff peaks form-- about 5-7 minutes.
Using a fine mesh sieve, sift the powdered sugar and almond flour into the egg white mixture.
Gently fold the dry ingredients into the egg whites by making a J shape with your spatula.
Once all of the dry ingredients have been incorporated, continue to fold the meringue but this time gently smush the batter against the sides of the bowl before folding it back together.Continue to smush and fold the meringue a few times before testing to see if it has reached the Figure 8 stage. The meringue is ready to pipe when you can draw a figure 8 without the stream breaking.
Transfer the meringue to a piping bag fitted with a small round tip (Wilton #10-12). Pipe 1” macaron shells onto the tray, making sure to pipe them at least 2” apart.
Once you’ve finished piping the first tray, hold it about 6 inches off the counter and drop it straight down. Drop the tray another 5-6 times, or until it looks like any large air bubbles have popped. Repeat with the second tray.
Set the trays aside to rest for 25 minutes, or until they have developed a skin. They’re ready to bake when you can touch them gently without the meringue sticking to your finger.
While the macarons rest, preheat your oven to 325F.
Bake the macarons, one tray at a time, for 13 minutes. If your oven has hot spots, make sure to turn the tray halfway through baking so the feet rise evenly.
Allow the macarons to cool to room temperature on the pan. Trying to remove the macarons while still warm may result in sticking.
For the Filling
In a medium-sized heat proof bowl, whisk together the granulated sugar, cornstarch, and egg yolk until it is pale yellow in color-- about 1 minute.
Add the milk and vanilla bean paste to a small saucepan set over medium heat. Bring it just to a simmer (it should just be starting to bubble around the edges).
Slowly pour the milk mixture into the egg mixture, whisking constantly. Make sure to pour slowly and don’t stop whisking-- you don’t want scrambled eggs!
Once all of the milk has been added, pour the mixture back into the saucepan and cook over medium heat for an additional 1-2 minutes, or until the mixture starts to thicken.
Once the custard can coat the back of a spoon, remove it from the heat and whisk in the butter.
Transfer it to an airtight container and cover the top with plastic wrap (make sure it’s touching the custard so it doesn’t form a skin).
Place the custard in the fridge to cool completely before transferring it to a piping bag.
For the Chocolate Ganache
Add the cream and chocolate to a heatproof bowl and microwave for 1-2 minutes, whisking every 30 seconds.
Dip the top of half of the macaron shells into the ganache and allow them to set fully before continuing-- placing them in the fridge can help speed up this process.
Assemble and Age
Pipe a small dollop of pastry cream onto the flat side of a macaron that has not been dipped in chocolate. Place a chocolate dipped macaron shell on top and press down gently, just until the filling reaches the edges.
Repeat with the remaining macarons. Transfer the filling macarons to an airtight container and store them in the fridge overnight to soften. Bring them to room temperature and enjoy.
If you don’t have vanilla bean paste, you can substitute the caviar from one vanilla bean. In a pinch, vanilla extract can be substituted but the beans give it a little something extra!