These Blackberry Pistachio Crumble Bars are the ideal summertime treat - portable, perfectly sweet and salty, and even more delicious with a scoop of vanilla ice cream! They are incredibly simple to make - the base and crumble are made with a mixture of oats, flour, butter, brown sugar, pistachios, and vanilla, and the blackberries cook down to a jam-like consistency beneath the oat crumble topping.
Preheat your oven to 350℉. Spray an 8x8 square metal or glass baking dish with nonstick spray, line with parchment paper, and spray again. Set aside.
In a medium mixing bowl, whisk together the oats, flour, cinnamon, baking soda, and salt until combined. In a separate, large mixing bowl, whisk together the melted butter, brown sugar, and vanilla until smooth. Add the dry ingredients to the wet, mixing with a rubber spatula to combine. Fold in the chopped pistachios. Once incorporated, let the dough sit for 5 minutes, to allow the flour to absorb the wet ingredients. If the dough seems too loose after this time, add an additional tablespoon of flour at a time, until mixture resembles wet sand consistency. Set aside.
In a medium bowl, stir together the blackberries, cornstarch, sugar, lemon juice, and a pinch of salt until combined. If any of the blackberries are super large, you can cut them/mash them in half with a spoon to create a more uniform filling. Set aside.
Press a little over half of the base/crumble mixture into the bottom of the pan, to create an even and smooth "crust". Add the blackberry mixture evenly over the crust. Crumble the remaining base/crumble mixture over top. Bake for 35 minutes, or until crumble is golden brown, and the center is set. Finish with a couple pinches of flaky sea salt, if desired. Let cool completely before cutting into bars.
*To keep these bars gluten free, you can substitute Bob's Red Mill Gluten Free 1-to-1 Baking Flour for the all-purpose flour.
*Bars will keep in a sealed container the fridge for 2-3 days.