Preheat oven to 350°F. Line a 9×9-inch baking dish with parchment paper, including up the sides. Set aside.
In a medium bowl, whisk together flour, sugar, and salt. Pour in melted butter and stir until combined.
Press the mixture evenly into the lined baking dish. Bake for 12-15 minutes, or until lightly golden. Remove from the oven and set aside.
In a separate bowl, toss diced strawberries with sugar, cornstarch, lemon zest, lemon juice, and vanilla extract.
Let the strawberry filling sit for 5 minutes, then spread evenly over the baked crust.
In a medium bowl, whisk together flour, brown sugar, and salt for the crumble topping. Stir in melted butter until a crumbly dough forms. Sprinkle evenly over the strawberry layer.
Bake for 30-35 minutes, or until the topping is golden brown and the strawberries are bubbling.
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, cut into bars, and serve.
