1

Butter a 5 by 9-inch loaf pan and put a rectangle of parchment paper on the bottom. Flour the sides of the pan. Preheat the oven to 350°F. In a stand mixer with the paddle attachment, beat the butter, sugar, and salt on high speed for about 8 minutes, or until fluffy. Don’t be tempted to shorten the beating time or your cake will be heavy. Scrape the sides of the bowl with a rubber spatula every minute or so.

2

In a bowl, beat the eggs, milk, and vanilla. With the mixer on medium speed, add the egg-milk mixture to the butter mixture, one-third at a time. Wait until each addition is thoroughly incorporated before adding more.

3

The mixture will have the consistency of sour cream or small curd cottage cheese, depending on the temperature. Add the zests to the batter and beat for 30 seconds more. Turn off the mixer and add all the flour. Beat on low speed for about 5 seconds, or just long enough to mix in the flour with no leftover lumps. Scrape the sides of the bowl with a rubber spatula, then mix for 5 minutes more. Scrape the batter into the prepared loaf pan. Don’t smooth over the surface of the batter or it may lose some of its airiness-it will settle as it bakes.

4

Bake for about 1 hour and 15 minutes, or until a toothpick or knife inserted in the center comes out clean.

5

Let cool for 15 minutes before turning out of the pan. Serve sliced. The cake keeps for several days tightly wrapped in plastic.