Preheat the oven to 150°C (300°F) and line baking sheets with parchment paper.

In a bowl, sift together the almond flour and powdered sugar to remove lumps and ensure a smooth texture. Set aside.

In a clean mixing bowl, whip the aged egg whites using a whisk or electric mixer until foamy. Gradually add the granulated sugar and continue to whip until stiff peaks form.

If using food coloring, add a few drops to the egg mixture and gently fold until incorporated.

Carefully fold the sifted almond flour and powdered sugar mixture into the whipped egg whites in three additions. The batter should be smooth and slightly thick, able to form a 'figure-eight' without breaking.

Transfer the batter to a piping bag fitted with a round tip. Pipe small circles (about 3cm in diameter) onto the prepared baking sheets, leaving space between each macaron.

Tap the baking sheets gently on the counter to release any air bubbles and allow the shells to rest for 30-60 minutes until they form a skin that is not sticky to the touch.

Bake in the preheated oven for 15 minutes, rotating the sheets halfway through for even baking. Let them cool completely on the baking sheet.

Prepare the berry filling by blending the mixed berries until smooth. Strain if desired, to remove seeds.

In a bowl, beat the softened butter until creamy. Gradually add the powdered sugar and vanilla extract, mixing until smooth. Fold in the berry puree until evenly combined.

Once the macaron shells have cooled, pair them up and pipe a small dollop of the berry filling onto the flat side of one shell. Sandwich it with the other shell, pressing down gently.

Store the assembled macarons in an airtight container in the refrigerator for at least 24 hours to allow the flavors to meld.