Position a rack to the lower third of the oven and preheat it to 325°F. Spray a 9x5-inch light-colored metal loaf pan with nonstick cooking spray and line it with 2 perpendicular strips of parchment paper — 1 cut skinnier to fit lengthwise across the bottom and up the 2 short sides, 1 to fit crosswise and up the 2 longer sides. Cut the strips long enough to have a few inches of overhang on all sides.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and nutmeg.

In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium-high speed until creamy. Add the sugar and vanilla and beat until light and fluffy, about 3 minutes. Beat in 1/4 cup of the flour mixture. Beat in the eggs, 1 at a time. Reduce the mixer speed to low, and stir in the remaining flour mixture and buttermilk in 5 alternating additions, beginning and ending with the flour mixture. Finish folding the batter by hand to make sure everything is incorporated — the batter will be very thick.

Scrape the batter into the prepared pan and smooth the top. Bake until the loaf is golden with a couple of cracks on top, and a toothpick inserted into the center comes out clean, 60 to 75 minutes. Let cool in the pan set over a wire rack for 15 minutes. Use the parchment paper to lift the loaf from the pan. Let rest for another 30 minutes.

When the loaf is cool and firm enough to handle, but still slightly warm, sift the powdered sugar all over a large rimmed baking sheet (keep the sieve handy). Peel the parchment from the cake. Gently turn the loaf over in 1 hand, using part of your forearm to support it. Using a pastry brush, brush the bottom of the cake with some of the melted butter. Carefully set the loaf, right-side up, in the powdered sugar. From there, brush the long sides with the butter, turning the cake from side to side to coat in sugar, then brush and coat the short sides. Lastly, brush the top with the butter, grab a handful or two of sugar from the tray, deposit into the sieve, and sift sugar generously over the top of the loaf. Roll the entire loaf in sugar once more so that it resembles a giant powdered sugar donut. Carefully transfer the cake to a wire rack to cool completely before slicing and serving, touching up the loaf with a quick sifting of sugar as needed.