Sift together flour, baking powder, baking soda, and salt.
In a large mixing bowl, combine milk, eggs, sugar, zucchini, and melted butter; mix well. Using a large spoon or spatula, gradually stir in flour mixture just until combined. Do not overmix.
Pour batter into prepared pan and bake 45 to 50 minutes, or until a toothpick inserted into the center comes out clean. Cool completely on a wire rack. Wrap airtight and keep in cool spot up to 4 days.
To make croutons: Preheat oven to 400°. Cut five 1/4-inch-thick slices of bread. Trim off crusts and cut into 1/4-inch cubes. Spread on a baking sheet in a single layer. Bake until toasted and golden on edges, 10 to 12 minutes.