Step 1: Preheat your oven to 375°F (190°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a large bowl, combine the all-purpose flour, granulated sugar, light brown sugar, and salt. Add the cold, cubed unsalted butter. Use your fingers or a pastry blender to cut in the butter until the mixture resembles coarse crumbs. Do not overmix; you want distinct crumbs.
Step 3: Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan to form the crust. Bake for 12-15 minutes, or until lightly golden.
Step 4: While the crust bakes, prepare the cherry filling. In a medium bowl, gently toss the pitted fresh cherries with granulated sugar, lemon juice, cornstarch, and almond extract. Ensure the cornstarch is evenly distributed.
Step 5: Once the crust is lightly baked, remove it from the oven. Pour the cherry filling evenly over the warm crust. Distribute the remaining one-third of the crumble mixture over the cherry layer.
Step 6: Return the pan to the oven and bake for another 35-45 minutes, or until the crumble topping is golden brown and the cherry filling is bubbling. The bubbling indicates the cornstarch has activated, thickening the filling.
Step 7: Allow the bars to cool completely on a wire rack before cutting into squares. Cooling completely is essential for clean cuts. You can refrigerate for an hour to firm up further before slicing.