Ingredients serves 8

4 eggs, yolks and whites separated
Unsalted butter (same weight as the four eggs without shells)
Caster or granulated sugar (same weight as the four eggs without shells)
Plain (all-purpose) flour, sifted (same weight as the four eggs without shells)
Zest 1 organic orange or lemon, or 1 vanilla pod / 1 tsp vanilla extract
Pinch salt

Extra butter and flour for cake tin

Method

Melt the butter, add the orange or lemon zest (if using) and set aside to cool. Whisk together the egg yolks and sugar in a mixing bowl. Stir in the melted butter and the vanilla (if using). Fold in the flour. Whisk the egg whites with the pinch of salt until they hold firm peaks. Fold into the cake mixture.

Butter and flour the loaf pan. Preheat the oven to 180°C/355°F (fan oven) or 200°C/390°F (conventional oven). Bake for 15 minutes then reduce the temperature to 160°C/320°F (fan oven) or 180°C/355°F (conventional oven). Bake for a further 25-30 minutes, until a knife inserted comes out clean.

Your cake will keep at room temperature for 3-4 days. It can also be frozen for up to a month.