Preheat oven to 350°F. Butter a 12 X 16-inch rimmed baking sheet.
In a large bowl, whisk together flour, salt, baking soda, baking powder and cinnamon. Set aside.
In a mixing bowl, cream butter and sugar until light and fluffy.
Beat in eggs. Blend in flour mixture along with zucchini and vanilla.
Spread mixture into the prepared pan and sprinkle chopped nuts evenly over the top.
Bake for 25-30 minutes or until golden brown.
In a small bowl, stir powdered sugar to break up lumps. Blend in butter, lemon zest and juice to form a thin icing.
Spread icing thinly over cake in the pan while it is still warm. Score cake into pieces and allow to cool.
Note: Cut into 2¾" x 3" pieces