Preheat the oven to 350°. Spray three 8-by-4-inch loaf pans (disposable are fine) with vegetable oil cooking spray, dust with flour and arrange them on a baking sheet.
In a medium bowl, whisk the cake flour with the baking powder, baking soda, salt and cinnamon. In the bowl of a stand mixer fitted with the paddle, beat the butter with the sugar and orange zest at medium-high speed until fluffy, about 4 minutes. Add the eggs one at a time, beating well between additions and scraping down the side of the bowl. Beat in the sour cream and vanilla. At low speed, beat in the dry ingredients in 3 additions, alternating with the buttermilk and beginning and ending with the dry ingredients; gently fold just until blended.
Scrape the batter into the prepared pans and smooth the tops; gently tap once to release any air. Bake the loaves for about 45 minutes, rotating the baking sheet halfway through, until a cake tester inserted in the center of each loaf comes out with a few moist crumbs attached.
Transfer the loaves to a rack for 20 minutes, then turn them out onto the rack and set them right side up to cool. Dust with confectioners' sugar, slice and serve.