Start by creaming the butter, sugar, and vanilla until light and fluffy. This creates a soft, tender texture. Add flour and salt, mixing just until combined. Press the dough evenly into a lined baking pan and chill before baking.
Bake the chilled crust in a preheated 350°F oven. It should turn lightly golden after about 15 minutes. Let it cool slightly before adding the filling.
Steep dried hibiscus in fresh lemon juice until the liquid turns deep pink. Strain out the solids. In a bowl, whisk together the hibiscus-lemon mixture, eggs, sugar, and flour until smooth.
Pour the filling over the warm crust. Return the pan to the oven and bake until the center is set. Once baked, allow the bars to cool completely before slicing. Dust with confectioner’s sugar before serving.