Heat the oven to 350°F. Line an 8x8-inch square baking pan with 2 sheets of parchment paper, positioning them perpendicular to each other so that the parchment hangs over all 4 sides of the pan to form a sling.

Stir 1 ½ sticks melted unsalted butter, 1 teaspoon vanilla extract, and ¾ teaspoon kosher salt together in a large bowl until well-combined. Stir in 2 cups all-purpose flour, ½ cup granulated sugar, and ¾ teaspoon baking powder until a soft dough forms and no dry streaks remain.

Reserve ¾ cup of the dough. Transfer the remaining dough to the baking pan, and press it into an even layer with your hands or a flat-bottomed measuring cup (reserve the bowl).

Bake until light golden-brown and the top appears dry, 17 to 20 minutes. Let cool slightly, 8 to 10 minutes. Meanwhile, make the pineapple filling.

Increase the oven temperature to 375°F. Drain 1 (20-ounce) can crushed pineapple and gently press against the strainer to remove as much liquid as possible (reserve the juice for another use).

Stir the pineapple, ½ cup granulated sugar, 3 tablespoons cornstarch, and ¼ teaspoon kosher salt together in the now-empty bowl (no need to clean) until combined.

Pour over the slightly cooled crust and spread into an even layer. Crumble the reserved dough evenly over the filling.

Bake until the top is golden-brown and the filling is set, 30 to 35 minutes. Place the pan on a wire rack and let cool completely, about 2 hours.

To serve, grasp the excess parchment and remove the slab from the pan to a cutting board. Dust with powdered sugar and cut into 16 pieces.