Sift powdered sugar and almond flour together into a large bowl. Process the mixture in a food processor for 30 seconds, then sift again. Repeat two more times, discarding any coarse bits left behind.

In a clean mixing bowl, beat the aged egg whites until foamy. Add cream of tartar and continue whisking while slowly adding granulated sugar. Beat until stiff peaks form. Add vanilla and food coloring at soft peak stage.

Gently fold one-third of the dry mixture into the meringue. Add the rest and continue folding using circular motions. Stop when the batter flows like lava and forms a figure-8 without breaking.

Transfer batter to a piping bag fitted with a round tip. Pipe 1-inch rounds onto parchment-lined baking sheets. Tap the sheets firmly to release air bubbles. Rest the shells for 40–60 minutes until a skin forms.

Bake in a preheated 300°F oven for 12–15 minutes, rotating trays halfway. Let macaroons cool completely before removing from parchment.

To make the filling, beat butter until creamy. In a saucepan, heat sugar and water until dissolved and boiling. Slowly drizzle the syrup into the egg yolks while beating until thick. Mix in vanilla and salt, then add butter until smooth.

Pipe buttercream onto the flat side of half the shells. Top with remaining shells and press gently to spread the filling to the edges.

Refrigerate assembled macaroons for 24–48 hours before serving for best texture. Bring to room temperature before enjoying.