In a saucepan, combine the blackberries, granulated sugar, lemon juice, cornstarch, and water.

Cook over medium heat, stirring occasionally, until the mixture thickens and the berries break down.

Remove from heat and strain through a fine mesh sieve to remove seeds. Allow the mixture to cool completely.

Sift together the almond flour and powdered sugar into a bowl.

In a separate bowl, whisk the egg whites until foamy.

Gradually add the granulated sugar and cream of tartar, whisking until stiff peaks form.

Gently fold the dry ingredients into the meringue in batches until the batter flows like lava.

Transfer the batter to a piping bag fitted with a round tip and pipe small circles onto baking sheets lined with silicone mats or parchment paper.

Tap the baking sheets firmly on the counter to release air bubbles and let the piped shells sit at room temperature until a skin forms (about 30 minutes).

Preheat the oven to 300°F (150°C) and bake the shells for 15-18 minutes, rotating sheets halfway through.

Allow the shells to cool completely before removing them from the baking sheets.

Pipe a small amount of blackberry filling onto half of the macaron shells.

Pipe a swirl of buttercream over the filling and top with another macaron shell.

Refrigerate the assembled macarons for at least 24 hours to allow the flavors to mature.