01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing overhang for easy removal.
02 - Pulse pistachios in a food processor until finely ground. Add flour, sugar, and salt; pulse to combine. Add cold butter and pulse until mixture resembles coarse crumbs. Press firmly and evenly into pan bottom.
03 - Bake for 12-15 minutes until lightly golden.
04 - Combine blackberries, sugar, and lemon juice in a saucepan over medium heat. Cook, stirring and mashing, for 3-4 minutes until juicy. Dissolve cornstarch in 1 tablespoon water, add to pan, and cook 1-2 minutes until thickened.
05 - Beat cream cheese and sugar until smooth. Add egg and vanilla, mixing until fully combined.
06 - Spread blackberry mixture evenly over baked crust. Pour cream cheese layer over berries and smooth top.
07 - Bake for 20-22 minutes until center is just set. Cool completely, then refrigerate at least 2 hours before slicing.
08 - Top with chopped pistachios and fresh blackberries if desired. Cut into bars and serve chilled.