01 - Preheat the 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 the prepared pan bottom. Bake for 12–15 minutes until lightly golden.
03 - Combine blackberries, sugar, and lemon juice in a small saucepan over medium heat. Cook, 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 the baked crust. Pour cream cheese layer over berries and gently smooth the top.
06 - Bake for 20–22 minutes until the center is just set. Cool completely in the pan, then refrigerate for at least 2 hours before slicing.
07 - Sprinkle with chopped pistachios and fresh blackberries if desired. Cut into bars and serve chilled.