Preheat the oven to 350°F (175°C). In a bowl, whisk together flour, sugar, and salt. Cut in butter until coarse crumbs form. Add egg yolk and ice water; mix until dough forms. Press into a greased baking pan and pre-bake for 15-20 minutes until golden.

For the filling, combine blackberries and sugar in a saucepan over medium heat. Cook until juices release, about 5-7 minutes. Stir in lemon juice and cornstarch; cook until thickened, about 1-2 minutes. Remove from heat and let cool.

For the topping, whisk together brown sugar, baking powder, and flour. Cut in butter until crumbly, then mix in egg and vanilla. Fold in pistachios.

Remove crust from the oven, pour blackberry filling on top, spreading evenly. Add pistachio topping, pressing down gently. Bake for 30-35 minutes until topping is golden and filling is bubbly.

Let cool completely in the pan before slicing into bars.