Follow these easy steps to create your delectable Blackberry Pistachio Dream Bars:

1. Preheat the Oven: Preheat your oven to 350°F (175°C) and line an 8×8-inch baking dish with parchment paper, leaving some overhang for easy removal.
2. Prepare the Crust: In a mixing bowl, combine the all-purpose flour, rolled oats, brown sugar, salt, and baking powder.
3. Mix the Wet Ingredients: In a separate bowl, mix together the melted butter and vanilla extract until combined.
4. Combine Mixtures: Add the wet ingredients to the dry ingredient mixture and stir until a crumbly dough forms.
5. Press the Crust: Firmly press about two-thirds of the mixture into the bottom of your prepared baking dish. Set aside the remaining mixture for topping.
6. Prepare the Blackberry Filling: In another bowl, gently toss the fresh blackberries with granulated sugar and lemon juice.
7. Layer the Filling: Evenly spread the blackberry mixture over the pressed crust.
8. Add Toppings: Sprinkle the remaining crumb mixture over the blackberries, then add the chopped pistachios on top.
9. Bake: Bake in the preheated oven for 30-35 minutes, or until the top is golden and the filling is bubbling.
10. Cool and Slice: Allow the bars to cool in the pan for about 1 hour. Use the parchment overhang to lift them out, then slice into squares.
11. Dust with Powdered Sugar: If desired, dust the tops with powdered sugar for an elegant finish before serving.