In a saucepan, combine fresh blackberries, granulated sugar, lemon juice, cornstarch, and a splash of water. Cook over medium heat while stirring gently until the mixture thickens, about 8-10 minutes. Once thickened, strain the mixture through a fine sieve to remove seeds, and let it cool completely in the fridge for at least 30 minutes.

Sift together almond flour and powdered sugar into a mixing bowl to remove lumps and combine evenly. In a separate bowl, whisk aged egg whites until foamy, then gradually add granulated sugar and cream of tartar, whisking until stiff peaks form—about 5-7 minutes.

Gently fold the sifted dry ingredients into the meringue in three additions using a spatula. Add purple gel food coloring at this stage, folding just until the color is evenly distributed.

Transfer the batter into a piping bag fitted with a round tip. Pipe 1.5-inch circles onto lined baking sheets, spaced an inch apart. Tap the sheets to release air bubbles. Allow to sit until a skin forms, about 30-60 minutes.

Preheat your oven to 300°F (150°C). Bake the macaron shells for 15-18 minutes, rotating halfway through. They are done when they’ve risen and have a smooth surface. Allow to cool completely.

In a mixing bowl, beat unsalted butter until creamy, about 2-3 minutes. Gradually add powdered sugar, mixing until smooth and fluffy. Optionally, mix in some blackberry filling.

Pipe a dollop of blackberry filling onto the flat side of half the macaron shells. Top with a swirl of buttercream and another shell to create a sandwich.

Refrigerate the assembled macarons for at least 24 hours for flavors to meld and shells to soften. Serve chilled or at room temperature.