In a large bowl, cream together the butter and sugar until smooth and pale. Add the egg, yolk, juice and zest and mix until well combined.
Sprinkle the flour and salt over the wet ingredients and mix on low until well combined - (do not over mix)
Pour batter into a greased 8x8 baking pan and bake at 350 for 25 minutes, or until the edges are just golden and the center of the pan is set. Cool bars before glazing.
For the glaze:
Stir the powdered sugar together with the lemon juice (1 tablespoon to start, adding more if needed) until a smooth consistency is reached.
Spread glaze over the bars and top with lemon zest.