Grease 11x17 rimmed baking sheet with non-stick cooking spray.
In a large bowl, whisk together sugar, flour, baking soda and salt.
Stir in melted butter and boiling water and lemon juice.
Mix in vanilla, buttermilk, eggs and lemon zest. Stir until smooth.
Pour batter into baking sheet and bake for 15-17 minutes. (Until toothpick inserted comes out clean.)
Cool cake for 15 minutes.
In a medium bowl, stir together melted butter, powdered sugar, milk, vanilla and lemon juice. Stir until smooth.
Spread frosting on cake.
Top with extra lemon zest for garnish.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.