1. Preheat your oven to 300°F (150°C). Line a baking sheet with parchment paper.
2. Sift the powdered sugar and almond flour together into a bowl.
3. In a separate bowl, beat the egg whites until foamy. Gradually add the granulated sugar and continue to beat until stiff peaks form. Add the vanilla extract and salt.
4. Gently fold the dry ingredients into the egg whites until well combined.
5. Transfer the batter to a piping bag and pipe small circles onto the prepared baking sheet.
6. Tap the baking sheet on the counter to release any air bubbles. Let the macarons sit at room temperature for 30 minutes to form a skin.
7. Bake for 15 minutes. Allow to cool completely before removing from the baking sheet.
8. For the filling, beat the butter until creamy. Gradually add the powdered sugar, vanilla extract, and milk until smooth.
9. Pipe the filling onto half of the macarons and sandwich with the remaining macarons.