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