• Preheat your oven to 375℉. Line a 9x13-inch tray with parchment paper.
• Prepare the sour cherries. Wash and pat the cherries dry. If using fresh cherries, you'll need to pit the cherries. Use a toothpick or cherry pitter. Once pitted, set the sour cherries aside.
• Cut the butter into tablespoons and set aside.
  1 cup unsalted butter, chilled and cubed
• In a large bowl, add the flour, brown sugar, granulated sugar, baking powder and salt and whisk together. Set aside.
  3 cups all-purpose flour, ½ cup brown sugar, ½ cup granulated sugar, 1 tsp baking powder, ½ tsp salt
• In a separate bowl, whisk together the egg and vanilla extract.
  1 tsp vanilla extract, 1 large egg, room temperature
• Add the egg and vanilla mixture and butter cubes into the large mixing bowl with the dry ingredients. Use forks, pastry cutter or your hands to bring the mixture together until it looks like large crumbs. You don't need to knead this into a firm ball of dough.
  1 cup unsalted butter, chilled and cubed
• Press ½ of the dough mixture into the baking tray. (Half of the mixture is about 428 grams.)

Sour Cherry Filling

• In a large bowl, add the sugar, lemon juice, and cornstarch. Stir the mixture together. Add the sour cherries and evenly coat them with the cornstarch mixture.
  4 cups sour cherries; washed, drained, pitted, and patted dry (or frozen cherries that are thawed and patted dry), ½ cup granulated sugar, 1 tbsp fresh lemon juice, 1 tbsp cornstarch
• Pour the cherries on top of the cookie base, spreading them out evenly. The cherries won't cover every space, but that's okay, the crumble will!
• Take the leftover cookie mixture and sprinkle it over the cherries. This doesn't have to be perfect, just crumble the bigger pieces in-between your hands if needed.
• Bake for 35 - 38 minutes until the top looks golden brown.  
• Cool the bars for 1-2 hours before cutting into 15 medium sized bars.