For the filling: In a large mixing bowl add the lemon zest and granulated sugar. Stir well to combine. Add the lemon juice, flour, baking powder, and beaten eggs. Mix until well combined. Pour mixture over baked crust while still hot. Bake at 350 degrees for 20 minutes, or until the lemon filling is set. It should not jiggle in the center. Remove from oven and cool for ten minutes. Use a strainer to sprinkle with the top confectioner's sugar. Let the bars cool at room temperature, then chill before cutting. Run a sharp knife around the edge of pan, then cut into squares. Add additional confectioner's sugar just before serving.