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

In a mixing bowl, sift together the almond flour and powdered sugar to eliminate any lumps.

In a separate clean bowl, whip the egg whites with the cream of tartar until soft peaks form. Gradually add the granulated sugar while continuing to whip until you achieve stiff peaks and the mixture is glossy.

Fold in the vanilla extract and food coloring, if using. Gently combine the dry ingredients into the whipped egg whites using a rubber spatula, being careful not to overmix.

Transfer the batter into a piping bag fitted with a round tip and pipe uniform circles onto the prepared baking sheets, spacing them about 2 inches apart.

Tap the baking sheets gently on the counter to release any air bubbles. Let them sit for about 30 minutes to form a skin.

Bake in the preheated oven for 15 minutes, or until the macarons are firm to the touch. They should not brown. Remove from the oven and allow to cool completely on the sheets.

While the macarons cool, prepare the berry filling by pureeing the mixed berries until smooth and straining through a fine mesh sieve to remove seeds.

In a mixing bowl, cream the softened butter and sugar until light and fluffy, then add the berry puree and lemon juice, mixing until well combined.

Once the macarons are cool, pipe a generous dollop of the berry filling onto the flat side of one macaron and sandwich with another.

Refrigerate the assembled macarons for at least an hour to allow the flavors to meld before serving.