Preheat the oven to 350°F (175°C). In a mixing bowl, whisk together flour, sugar, and salt. Cut in butter until crumbly. Mix in egg yolk and ice water until a dough forms. Press into a greased 9x13 inch baking pan and pre-bake for 15-20 minutes.

In a saucepan, combine blackberries and sugar over medium heat. Cook for 5-7 minutes until softened. Stir in lemon juice and cornstarch, cooking for an additional 2-3 minutes until thickened. Cool slightly and pour over the crust.

In a bowl, whisk together brown sugar, baking powder, and flour. Cut in cold butter until crumbly. Mix in a beaten egg and vanilla, fold in pistachios.

Pour blackberry filling over the crust, then add the pistachio topping. Bake for 30-35 minutes or until golden brown and bubbly.

Allow bars to cool completely in the pan for 1-2 hours before slicing into squares or rectangles.