Preheat oven to 325℉. Brush a 14-cup Bundt pan with melted butter.
In a medium-sized bowl, whisk together flour, baking powder, and salt.
Grate zucchini on the large holes of a shredder or box grater. Squeeze zucchini dry in a clean kitchen towel (You will need about 2½ cups total). In a large bowl, stir together eggs, granulated sugar, melted butter, zucchini, orange zest, and juice. Stir in dry ingredients.
Transfer batter to prepared pan and bake about 60 minutes, or until cake tester (toothpick) comes out clean.
Transfer to a wire rack to cool for 10 minutes, then gently remove from the pan. Allow to cool completely before adding the glaze.
Whisk together all ingredients for the glaze. Thin the glaze with whole milk if necessary (don't add more than a tsp at a time). Drizzle glaze over the cake and serve.
