Preheat oven to 325.
In a medium bowl, stir together the eggs, oil, zucchini, sugar and vanilla until well blended.
In a separate bowl, combine the flour, baking soda, baking powder, salt, and cinnamon.
Add the dry ingredients to the zucchini mixture and stir just until combined.
Divide the batter evenly into THREE 9″ cake pans (well-greased or non-stick). If you don’t have three pans, bake the cake in batches.
Bake until the cake is light brown and set in the middle – 22 to 25 minutes.
Turn cakes out onto a rack to cool.
In a medium sauce pan, combine the brown sugar and butter and cook over medium heat, stirring constantly, until the butter has melted and the sugar has dissolved – about 2 to 3 minutes.
Pour in the milk. Continue to stir and bring to a boil.
Remove the pan from heat and stir in the Mapleline flavoring. Set aside to cool slightly.
Place the powdered sugar in a a large bowl. Pour the butter mixture over the powdered sugar and mix by hand for 2 – 3 minutes until the icing is smooth. Use the icing while it is still warm.