• Preheat oven to 350°F (175°C). Line a 9×9-inch baking dish with parchment paper, leaving overhang for easy removal. Lightly grease if needed.

Crust

• In a medium bowl, whisk together flour, sugar, and salt. Stir in melted butter until fully combined. The mixture will be crumbly but should hold together when pressed.
• Press evenly into the prepared pan.
• Bake for 12 to 15 minutes, until lightly golden. Set aside.

Berry Filling

• In a large bowl, gently toss strawberries and blueberries with sugar, cornstarch, lemon zest, lemon juice, and vanilla.
• Let sit for 5 minutes to allow juices to release and lightly thicken, then spread over the baked crust.

Crumble Topping

• In a medium bowl, whisk together flour, brown sugar, and salt.
• Stir in melted butter until crumbly clumps form.
• Sprinkle crumble topping evenly over the berry filling.
• Bake for 30 to 35 minutes, until the topping is golden and the filling is bubbling at the edges. Tent loosely with foil if the top browns too quickly.
• Transfer to a wire rack and let cool completely (about 1 hour), then refrigerate until chilled, about another hour.
• Once chilled, dust with confectioners’ sugar, then slice into bars. If desired, top with fresh berries or a dollop of whipped topping before serving.