Preheat your oven to 180C/350F and greaseproof a baking tin.
Toss the egg yolks, butter, sugar, vanilla extract, salt and lemon zest into a bowl and whip on medium speed until pale and fluffy, approximately 3 minutes.
In a separate bowl, stir the flour and baking powder together until combined and toss into the egg yolk mixture.
Whip on a low setting until combined and no dry portions remain in the edges or base of the bowl.
In a large separate bowl, whip the egg whites on medium speed, until stiff, approximately 8 minutes.
Add ½ the egg yolk mixture into the egg whites and fold slowly until the mixture is smooth.
Repeat with the remaining egg yolk mixture until your batter is smooth.
Pour into your baking tin and smooth out evenly.
Press the halved apricots tightly into your cake tin, cut side up.
Place in the oven and bake for 40 minutes.
Remove and allow to cool fully.
Optional: Stir the apricot jam and water together and brush over the apricots whilst hot.
Slice and serve.
Enjoy!