Zest the orange with a zester or grater.
Finely mince the orange zest using a knife and set the zest aside.
Juice the orange with your hands or a juicer.
Crack the eggs into a strainer skimmer with a bowl underneath to separate the egg yolks and egg whites.
Whisk the egg whites in a separate bowl until fluffy.
Gradually add the sugar.
Then, add the egg yolks one at a time while whisking.
Sift the plain flour and sea salt into the egg mixture.
Fold the ingredients together with a spatula until well incorporated.
In a bowl, combine melted butter, orange juice, and orange zest.
Then, add some of the cake batter into the bowl and mix it well.
Once mixed, pour the orange mixture into the batter and fold gently until well incorporated.
Grease your baking pan and line it with parchment paper.
Pour in the cake batter.
Bake the cake in the preheated oven to bake at 160°F for 25 to 30 minutes or until done.
Let it cool, and serve it for breakfast or dessert!