Follow these simple steps to create your Easy Blackberry Pistachio Dessert Bars:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper.
2. Make the Base: In a bowl, combine flour, sugar, melted butter, and salt. Mix until a crumbly dough forms. Press this mixture evenly into the bottom of the prepared pan.
3. Pre-Bake the Base: Bake the base in the preheated oven for about 15 minutes or until lightly golden. Remove from the oven and let it cool slightly.
4. Prepare the Blackberry Layer: In a saucepan, combine blackberries, sugar, cornstarch, and lemon juice. Cook over medium heat until the mixture thickens and the blackberries break down, about 5-7 minutes. Remove from heat and let cool slightly.
5. Spread the Blackberry Layer: Evenly spread the blackberry mixture over the cooled base, ensuring it covers the entire surface.
6. Make the Pistachio Topping: In a separate bowl, mix finely chopped pistachios, rolled oats, brown sugar, melted butter, and cinnamon until well combined.
7. Add the Pistachio Layer: Sprinkle the pistachio mixture over the blackberry layer, pressing it down gently to help it adhere.
8. Bake Again: Return the dessert to the oven and bake for an additional 15-20 minutes, or until the topping is golden brown.
9. Cool the Bars: Once baked, remove from the oven and let cool in the pan for about 30 minutes. Then, lift the bars out using the parchment paper and transfer them to a wire rack to cool completely.
10. Slice and Serve: Once cool, slice into squares or rectangles for serving.