Position a rack in the lower third of the oven and heat the oven to 350°F. Butter and then flour the bottom and sides of a 6-cup (8-1/2 x 4-1/2-inch or 9 x 5-inch) loaf pan, tapping out any excess flour.
In a large bowl, combine the flour, sugar, baking powder, salt, and cinnamon. Whisk until well blended. Stir in the pistachios.
In a medium bowl, combine the milk, eggs, and vanilla. Add the zucchini and whisk until blended. Pour the wet ingredients over the dry ingredients. Add the melted butter. Using a silicone spatula, gently fold until blended.
Scrape the batter into the prepared pan and spread evenly.
Bake, rotating halfway through, until the top is golden and a wooden skewer or toothpick inserted in the center comes out clean, 50 to 55 minutes.
Let cool in the pan on a rack for 30 minutes. Invert the bread and remove the pan, turn the bread right side up, and let cool completely. Dust with confectioners’ sugar. Store, covered, at room temperature for up to 2 days.