Preheat your oven to 180C/350F and lightly greaseproof a loaf tin, lining the base with greaseproof paper.
Place the sugar and orange zest into a bowl and rub together to coat the zest thoroughly with sugar and draw out the oils in the peel.
Toss in the butter and beat until pale and fluffy, approximately 3 minutes with an electric beater.
Add the egg yolks and whip until combined, approximately 1 minute.
Add in 1 yolk and beat until well combined, approximately 1 minute.
Repeat with each remaining egg yolk until well combined.
Stir the flour, baking powder and salt in a separate bowl.
Toss into the butter and egg yolks and whip on low until just combined and no dry portions remain in your bowl.
Whip the egg whites in a clean and dry bowl until stiff peaks form, approximately 9 minutes.
Fold the egg whites into the batter until smooth.
Pour evenly into the cake tins and place in the oven until golden, approximately 20 minutes or until the edges have pulled away from the tin and a skewer comes out clean.
Beat on low until just combined and no dry portions remain in your bowl.
Remove and allow to cool in the tin for 5 minutes before turning onto a cooling rack.