01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - In a food processor, pulse pistachios until finely ground. Add flour, sugar, and salt; pulse to combine. Add cold butter and pulse until the mixture resembles coarse crumbs. Press the mixture firmly and evenly into the bottom of the prepared pan. Bake for 12–15 minutes, until lightly golden.
03 - In a small saucepan, combine blackberries, sugar, and lemon juice. Cook over medium heat, stirring and mashing berries, until juicy (about 3–4 minutes). Dissolve cornstarch in 1 tablespoon of water, add to the pan, and cook for 1–2 minutes until thickened. Remove from heat and let cool slightly.
04 - In a bowl, beat cream cheese and sugar until smooth. Add egg and vanilla extract, mixing until combined.
05 - Once the crust is baked, spread the blackberry mixture evenly over it. Pour the cream cheese layer over the berries and gently smooth the top.
06 - Bake for 20–22 minutes, or until the center is just set. Cool completely in the pan, then refrigerate for at least 2 hours before slicing.
07 - Garnish with chopped pistachios and fresh blackberries, if desired. Cut into bars and serve chilled.