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 eggs and whip until combined, approximately 1 minute.
Stir the flour and baking powder together and add to the wet ingredients.
Beat on low until just combined and no dry portions remain in your bowl.
Spoon your batter into your loaf tin and spread out evenly.
Optional step: Place a thin line of butter into the center of your cake before baking (this helps it rise evenly in the center).
Place in the oven and bake until golden and slightly pulling away from the sides of the tin, approximately 55 minutes.
Remove and allow to cool in the tin for 5 minutes before turning onto a cooling rack.