Adjust rack to the middle position and preheat the oven to 325 degrees F. Coat an 8 1/2-by-4 1/2-inch loaf pan with cooking spray.
Whisk together the butter, oil, vanilla and eggs in a medium bowl. Fold in the zucchini.
Whisk together the flour, brown sugar, granulated sugar, baking powder, baking soda and salt in a large bowl. Gently stir the zucchini mixture into the flour mixture until just combined. Spoon the batter into the prepared pan, smoothing out the top with the back of a spoon.
Bake, rotating the pan about halfway through, until puffed and golden brown on top and a toothpick inserted into the center comes out clean, 1 hour to 1 hour 20 minutes.
Let cool completely in the pan on a wire rack.
When measuring flour, we spoon into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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