Preheat oven to 300°F. Line two baking sheets with parchment paper. Set aside.
Heat a small pot of water over medium heat until it steams. In the bowl of a stand mixer, combine egg whites and granulated sugar. Place bowl over steaming pot, creating a double boiler.
Whisk egg whites and sugar until sugar melts completely and egg whites become white and frothy, 1 to 1.5 minutes. Remove from heat and place bowl back onto standing mixer.
Fit standing mixer with whisk attachment and whisk on high speed for 2-3 minutes, until stiff peaks form. Congratulations, you just made a Swiss meringue!
Sift together almond flour and powdered sugar, then gently shake into meringue bowl. Drizzle in pistachio paste and green food coloring (if using).
Place bowl onto stand mixer fitted with a whisk attachment. Turn mixer to medium speed and whisk for 10 seconds. Stop mixer and use a spatula to scoop up some of the batter. If you can make a figure-eight with the dripping batter without the batter breaking, you can stop. But most likely you will need another 5-10 seconds. So repeat process, and try the figure-eight again. The best way to describe the perfect consistency is like slow moving lava. It should give, but only slightly, like in slow-motion.
Once your batter is the correct consistency, scoop it into a large pastry bag fitted with a 1 inch tip. Pipe silver dollar-sized circles onto prepared baking sheets, keeping about 1.5 inches between each meringue. Bang baking sheet on counter two times to remove any air bubbles. Sprinkle tops with a little sea salt as a garnish.
Here’s the fun part: you don’t need to let them air dry. Just pop them into the prepared oven for 13 minutes, turning around halfway through, then allow to cool completely before removing from baking sheets.
While the macarons are baking, make the ganache. In a small microwave-safe bowl, microwave the heavy cream for 45 seconds. Remove, then stir in white chocolate, pistachio paste, and green food coloring (if using).
Scoop mixture into a piping bag fitted with a 1/2 inch tip, and place in the fridge to harden (about 20 minutes).
Once you’re ready, pipe the ganache onto a macaron and sandwich with another macaron. Repeat with all macarons.