In a large bowl, use an electric mixer to combine the flour, butter and powdered sugar. Press the crust into a 9x13-inch pan. Bake for 20 minutes, or until light golden brown.
PREPARE THE FILLING:
In a large bowl, use a whisk to combine all of the filling ingredients. Pour the filling over the baked crust and bake for an additional 25 minutes. Let the filled lemon bars cool completely before adding the frosting.
ADD THE FROSTING:
In a large bowl, use an electric mixer to combine the frosting ingredients. Mix until smooth. Spread carefully over the cooled lemon bars. Sprinkle lemon zest on top, if desired. Refrigerate until firm, and then slice into squares and serve.