01 -
Heat your oven to 350°F (175°C). Grease a 9×9-inch pan or use parchment paper to make removal easier. Chop the pistachios in a food processor until fine but not powdery. Mix the chopped nuts, flour, sugar, baking powder, and salt in a big bowl. Add the soft butter and mix until everything sticks together. Mix in the oats. Push the mixture firmly into your pan. Bake it for 15-18 minutes until it's slightly golden and firm. Take it out and let it sit.
02 -
Grab a medium bowl and smash the blackberries with a fork to get their juice out. Mix in the sugar, eggs, vanilla, flour, salt, and baking powder until everything's well blended. Pour this blackberry mix over your baked base and spread it out evenly. Sprinkle what's left of your pistachio mixture on top, trying to cover everything evenly.
03 -
Put the pan back in the oven and let it cook for 30-35 minutes until the filling doesn't jiggle and the top looks golden brown.
04 -
Let everything cool completely in the pan on a wire rack before you cut it. This helps the bars hold together when you slice them. Cut into 16 squares when cool. You can top with extra pistachios or a bit of powdered sugar to make them look fancy.