Prepare the Blackberry Filling: In a medium saucepan, combine blackberries, granulated sugar, lemon juice, cornstarch, and water. Cook over medium heat for about 10 minutes, stirring frequently, until thickened and bubbling. Strain through a fine-mesh sieve to remove seeds and cool.

Make the Macaron Shells: Sift together almond flour and powdered sugar. Whisk egg whites until foamy. Gradually add granulated sugar and cream of tartar, beating until stiff peaks form, about 5-7 minutes.

Combine the Mixtures: Fold the almond flour mixture into the meringue until the batter flows like thick lava, being careful not to over-mix.

Pipe the Macaron Shells: Line baking sheets with parchment paper, pipe small circles onto sheets, tap to release air, and let sit at room temperature for 30 minutes.

Bake the Macaron Shells: Preheat oven to 300°F (150°C) and bake for 15-18 minutes until risen and shiny. Cool completely.

Make the Buttercream: Beat unsalted butter until creamy, gradually add powdered sugar, mixing on low, then whip until fluffy. Add vanilla if desired.

Assemble the Macarons: Pair up macaron shells, pipe blackberry filling onto half, top with buttercream, and sandwich together. Refrigerate for 24 hours before serving.

Serve and Enjoy: Your Delectable Blackberry Macarons are ready to indulge!