Bring a pot of water to the boil over medium heat and cook the zucchini until tender, about 5 minutes. Drain the zucchini in a colander set in the sink. Set aside to cool.
Preheat oven to 180 degrees C. Grease a 23x33cm baking dish.
Combine flour, sugar and very soft butter in a bowl until crumbly and well combined. Press the mixture into the bottom of the prepared baking dish.
Measure out 3 cups of cooked zucchini, and place into a blender. Pour in the evaporated milk and eggs, sugar, flour, butter and the vanilla essence to the zucchini. Pulse the blender several times to combine the ingredients, then blend until the filling mixture is pureed, about 1 minute. Pour the mixture over the crust.
Combine sugar with the cinnamon in a small bowl, sprinkle over the filling.
Bake until the middle of the dessert is set, about 45 minutes. Let cool, and chill in refrigerator before slicing.