Heat the oven to 350°F (175°C). Line a 9-by-13-inch baking pan with parchment paper, leaving overhang on two sides, and lightly grease the exposed sides.

Whisk together the flour, sugar, shredded coconut, and salt for the crumble. Stir in the melted, slightly cooled butter and vanilla until damp, coarse crumbs form.

Reserve approximately 1 to 1 1/4 cups of the crumble mixture for the topping. Press the remaining mixture firmly and evenly into the prepared pan.

Bake the crust for 18–20 minutes, until it looks set and the edges are just beginning to turn golden.

While the crust bakes, whisk the eggs and sugar until smooth. Whisk in the lemon juice, lemon zest, flour, and salt, then stir in the coconut without whipping the mixture excessively.

Pour the lemon filling over the hot crust. Sprinkle the reserved crumble mixture evenly over the surface.

Bake for another 23–30 minutes, until the filling is set around the edges and only slightly jiggles in the center. The crumble should be lightly golden.

Cool the bars completely in the pan, then refrigerate until firm. Lift them out using the parchment, dust with confectioners’ sugar if desired, and cut into squares.