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.