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 melted unsalted butter, vanilla extract, and kosher salt together in a large bowl until well-combined. Stir in all-purpose flour, granulated sugar, and 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.

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

Increase the oven temperature to 375°F. Drain the crushed pineapple and gently press against the strainer to remove as much liquid as possible.

Stir the pineapple, granulated sugar, cornstarch, and kosher salt together 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.