Preheat oven to 350° F. Lightly butter a 9- by 13-inch pan.

Combine 1 1/2 cups flour, powdered sugar and 1/2 teaspoon salt in a food processor. Coarsely chop butter and drop the pieces on top of dry ingredients. Pulse until butter has been evenly blended into the flour mixture and looks like coarse cornmeal.

Pour mixture into the prepared pan. Press firmly with your fingers across the entire bottom of the pan and about 1/2-inch up the sides.

Bake until pale golden brown, about 20 minutes. Set aside.

While crust bakes, prepare the filling. Warm lemon juice slightly in a small saucepan set over medium heat. Sprinkle hibiscus flowers on top of juice, stir gently and remove from the heat. Let steep for 10 minutes. Strain out flowers and stir in zest.

Whisk together juice and eggs in a bowl, then whisk in granulated sugar, remaining 1/4 cup flour and 1/8 teaspoon salt. Pour filling into crust.

Reduce oven temperature to 325° F. Bake until filling is fairly set, 25 to 30 minutes. Cool in the pan at least 1 hour and then cut into bars. Served chilled or at room temperature. Just before serving, sift powdered sugar on top. Will keep, refrigerated, for up to 1 week.