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 mixture resembles coarse crumbs. Press firmly and evenly into pan bottom. Bake for 12–15 minutes until lightly golden.
03 - Combine blackberries, sugar, and lemon juice in a small saucepan. Cook over medium heat, 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 and cool slightly.
04 - Beat cream cheese and sugar until smooth. Add egg and vanilla extract, mixing until fully combined.
05 - Spread blackberry mixture evenly over baked crust. Pour cream cheese layer over berries and gently smooth top. Bake for 20–22 minutes until center is just set.
06 - Cool completely in pan, then refrigerate at least 2 hours before slicing. Garnish with chopped pistachios and fresh blackberries if desired. Cut into bars and serve chilled.