Preheat oven to 350F. Grease an 8-inch square baking pan and line with parchment paper, letting it overhang on both sides.
In a medium bowl, combine flour, coconut, powdered sugar, cornstarch, and salt. Stir the oil into the flour mixture until fully incorporated. Place the dough in the prepared pan, and firmly press all the way to the sides. Bake the crust until it just starts to brown around the edges, 15 - 20 minutes.
In a separate bowl, whisk the sugar, cornstarch, and salt until well combined. Add the eggs, one at a time, whisking until each one is fully incorporated before adding the next. Add the lemon zest and juice; whisk until mixture is completely smooth.
Pour the filling over the hot crust. Bake until the filling is just set, 15 - 18 minutes. The filling should start to turn golden around the edges and the centre should be a bit jiggly (it will firm up as it cools). Let the squares cool completely to room temperature and then refrigerate for 2 hours before cutting.
Just before serving, dust the bars with powdered sugar and garnish with coconut.
Notes
Recipe from Simply Citrus by Marie Asselin, reprinted by permission of Gibbs Smith Yields 9 large bars or 12 small bars