Heat the oven to 375F. Coat a 9x5 loaf pan with butter and line it with parchment paper so that the ends of the paper hang down over sides.
Using either a hand mixer or a standing mixer, beat the egg whites on medium speed until foamy. Add a pinch of cream of tarter. Then beat until you see stiff peaks, stopping to check frequently. Set the beaten egg whites aside in the refrigerator until you are ready to use them.
Mix together the flour, salt, and baking powder in a medium bowl.
Measure the orange juice and zest and set aside.
Using your mixer, beat the butter on medium (to medium-high) until fluffy, about 2 minutes.
Add the sugar and continue beating for about 3 minutes. The mixture will be light colored and fluffy.
Add the orange zest and mix in. Then, with the mixer running, add the egg yolks one at a time, waiting a few second between each one. Continue beating for 30 seconds after they are all added.
Scrape down the mixer, then turn it on for a few seconds, then scrape it down again.
Add 1/3 of the flour mixture and beat on low until almost combined. Then add 1/2 of the orange juice and mix in. Continue with 1/3 of the flour, the remaining 1/2 of the orange juice, and the last 1/3 of flour. Stop with some streaks of flour remaining.
Scoop the egg whites into the mixing bowl. Using a large silicone spatula, gently fold into the batter until no white streaks remain.
Pour the batter into the prepared pan. Smooth the top. Bake at 375F for 10 minutes.
Reduce the oven temperature to 325F. Continue baking for 50-55 minutes, or until golden brown and cooked all the way through.
Let the cake rest for 10-15 minutes in the pan. Then lift it out with the parchment paper sides and move it to a cooling rack. Cool to room temperature.