Preheat your oven to 300°F (150°C) and line two baking sheets with Silpat mats or parchment paper.

In a medium bowl, sift together the almond flour and powdered sugar. Set aside.

In a stand mixer bowl, whip the egg whites on medium speed until foamy. Add a pinch of salt and gradually add the granulated sugar while increasing to high speed. Whip until stiff peaks form and the mixture turns glossy.

If using pink gel food coloring, add a small amount and gently fold it in until uniformly colored.

Fold the sifted almond flour and powdered sugar mixture into the whipped egg whites using a rubber spatula, being careful not to deflate the batter. The batter should flow like lava and form ribbons.

Transfer the batter to a piping bag fitted with a round tip. Pipe small circles (about 1.5 inches in diameter) onto the prepared baking sheets, leaving space in between.

Tap the baking sheets against the counter to release any air bubbles. Let the macarons sit at room temperature for about 30 minutes until a skin forms.

Bake the macarons in the preheated oven for 15-18 minutes, or until they are set and no longer sticky. Let them cool completely on the baking sheets.

While the macarons cool, prepare the berry buttercream filling. In a mixing bowl, combine the softened butter and powdered sugar. Beat until creamy, then add the mixed berry purée, vanilla extract, and salt. Beat until light and fluffy.

Once the macaron shells are cool, pair them up by size. Pipe a dollop of berry buttercream onto the flat side of one shell and sandwich it with its pair.

Allow the assembled macarons to rest in the refrigerator for about 24 hours to enhance the flavors. Serve chilled or at room temperature.