Preheat the Oven: Preheat your oven to 350°F (175°C). Line an 8x8-inch pan with aluminum foil and grease with melted butter.
Prepare the Blackberry Puree: Wash and blend fresh blackberries until smooth. Strain through a fine mesh sieve to remove seeds, aiming for 3/4 cup of puree.
Make the Shortbread Crust: In a bowl, combine the flour, sugar, and salt. Stir in the melted butter until no dry flour remains. Press the mixture evenly into the prepared pan and bake for 18-22 minutes, or until golden brown.
Cook the Blackberry Lemon Curd: While the crust bakes, combine the blackberry puree, lemon juice, and sugar in a saucepan. Heat over medium heat until the mixture comes to a simmer.
- Whisk in the eggs, egg yolks, and butter. Cook while stirring constantly until the mixture thickens and reaches 190°F.
- Strain the curd through a sieve to ensure a smooth texture, then pour over the baked shortbread crust.
Bake the Bars: Return the pan to the oven and bake for 13-15 minutes, or until the curd is set.
Optional Meringue Topping: In a heatproof bowl, combine egg whites and sugar. Set the bowl over a saucepan of simmering water, whisking constantly until the sugar dissolves and the mixture is warm to the touch.
- Remove from heat and beat the mixture with an electric mixer until stiff peaks form. Pipe or spread the meringue onto the cooled bars. Lightly torch or broil for a golden finish, if desired.
Cool and Serve: Let the bars cool to room temperature, then chill in the refrigerator for 1 hour. Slice into squares and serve.