Preheat the oven to 325 degrees F (165 degrees C).

Combine flour, butter, and sugar in a stand mixer fitted with the paddle attachment. Beat together until a soft dough forms. Press dough over the bottom and up the sides of a 9x12-inch baking pan.

Bake in the preheated oven until edges are light golden, 20 to 25 minutes.

While crust is baking, bring sugar and crushed pineapple to a boil in a small saucepan. Mix cornstarch with water and stir into the pineapple filling. Simmer until thickened, about 5 minutes. Remove from heat and cool slightly.

While filling is cooling, beat egg whites in a glass, metal, or ceramic bowl until stiff peaks form. Add slightly cooled pineapple mixture; whip on low speed until just combined. Give filling 1 more stir to ensure it is evenly mixed.

Pour filling into the baked crust. Return to the oven and bake until the topping is slightly golden brown, about 10 minutes. Cool before cutting into squares.