Mix crushed biscuits with melted butter and press into the bottom of a lined 8×8 inch baking pan. Chill for 20 minutes.
Blend ground pistachios with confectioners’ sugar and heavy cream until smooth. Spread over the biscuit base and chill for another 20 minutes.
Beat cream cheese with confectioners’ sugar and vanilla until creamy. Spread over the pistachio layer and chill.
Heat blackberries, granulated sugar, lemon juice, and cornstarch in a saucepan over medium heat. Cook until thickened. Cool slightly before spreading over the cream cheese layer. Chill until set.
Before serving, sprinkle with chopped pistachios and garnish with fresh blackberries.
