01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Pulse pistachios in 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. Bake for 12-15 minutes until lightly golden.
03 - Combine blackberries, sugar, and lemon juice in saucepan over medium heat. Cook, stirring and mashing berries, until juicy (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.
06 - Bake for 20-22 minutes until center is just set. Cool completely in pan, then refrigerate at least 2 hours before slicing.
07 - Garnish with chopped pistachios and fresh blackberries if desired. Cut into bars and serve chilled.