Line two baking sheets with parchment paper or silicone mats. Sift almond flour and 100g powdered sugar together twice, then pulse in a food processor for 30 seconds to create a fine powder. Set aside.
Beat the aged egg whites in a stand mixer on medium speed until foamy, about 1 minute. Gradually add granulated sugar one tablespoon at a time, then increase to high speed and beat until stiff, glossy peaks form, about 4-5 minutes. The meringue should stand straight up when you lift the whisk.
Add the almond flour mixture and salt to the meringue. Using a spatula, fold gently but deliberately, scraping down the sides and turning the bowl. Continue folding until the batter flows like lava and a ribbon dissolves back into the mixture within 10-15 seconds. This should take 35-50 folds.
Transfer batter to a piping bag fitted with a round tip. Pipe 1.5-inch circles onto prepared baking sheets, spacing them 1 inch apart. Tap sheets firmly against the counter 5-6 times to release air bubbles. Let rest at room temperature for 30-60 minutes until shells develop a dry skin that doesn't stick when lightly touched.
Preheat oven to 300°F (150°C). Bake one sheet at a time for 15-17 minutes, rotating halfway through. Shells are done when they don't wiggle when gently touched and have developed feet. Let cool completely on the baking sheet before removing.
Make the buttercream: Beat softened butter on medium-high speed for 3 minutes until pale and fluffy. Add powdered sugar, pistachio paste, ground pistachios, heavy cream, vanilla, and salt. Beat on low to combine, then increase to high and whip for 2-3 minutes until light and creamy.
Match shells into pairs of similar size. Pipe or spread buttercream onto the flat side of one shell, then gently sandwich with its pair, pressing just until the filling reaches the edges. Repeat with remaining shells.
Refrigerate filled macarons in an airtight container for 24 hours before serving to allow flavors to meld and shells to soften slightly. Bring to room temperature 20 minutes before enjoying for the best texture and flavor.