Set your oven to preheat at 350°F. Line an 8x8-inch baking pan with two sheets of aluminum foil, folded lengthwise to 8 inches wide. Fit one sheet along the bottom and up the sides, then place the second sheet perpendicular to the first. Brush the foil with melted butter.

Rinse the blackberries and process them in a food processor. Strain the puree through a fine mesh strainer, pressing with a spoon to extract about 3/4 cup of seedless puree.

For the Shortbread Crust:

Mix the flour, 1/2 cup sugar, and salt together. Add the melted butter and stir until the flour is fully incorporated. Press the dough into the prepared pan and bake for 18-22 minutes, until golden brown. Remove from the oven and pour the blackberry lemon curd over the warm crust.

For the Blackberry Lemon Curd:

While the crust bakes, combine the blackberry puree with lemon juice in a non-reactive saucepan. Stir in 1 cup of sugar, taste, and adjust sweetness with additional sugar if needed.

Place the pan over medium-low heat and add the whole eggs, yolks, and butter. Whisk constantly until the mixture thickens and reaches 190°F. Strain the curd into a bowl and pour it over the warm shortbread crust. Bake for 13-15 minutes, until slightly firm with a jiggle in the center. Cool to room temperature, then refrigerate for 2 hours before cutting into bars.

For the Meringue Topping (Optional):

Boil water and reduce to a simmer. Place the egg whites and 3/4 cup sugar in a mixer bowl over simmering water. Whisk until the mixture reaches 160°F. Transfer to the mixer, attach the whisk, and whip on medium-high speed until cool and stiff peaks form, about 5-7 minutes.

Pipe the meringue onto the chilled bars using a pastry bag with a star or round tip.