Actually, the recipe is a cake. But it makes perfect muffins. Especially because it's a dense and sturdy and moist cake filled with grated zucchini, lots of cocoa, and both butter and oil and eggs. Plus buttermilk gives it that muffin-like texture and crumb.
The muffins are perfect for this time of year with zucchini is at its peak and grows like crazy (like the giant zucchini behind the muffins).