Preheat oven at 150 degrees C (300 degrees F). Line paper cups in muffin pan.
In a bowl, stir Large Egg (4) yolks with a hand whisk and pour in Yogurt (200 gram) and stir till combined.
Sift in half of the Cake Flour (60 gram), stir well gently, followed by the remaining flour, mix well again.
Add Vanilla Extract (1/2 teaspoon), Lemon Juice (1 tablespoon) and Lemon (1), mix well. Pour in Butter (40 gram) and fold well with a rubber spatula.
Whisk egg whites at low speed till foamy and gradually add in Caster Sugar (75 gram) and continue to beat till peak forms at medium speed.
Fold ⅓ of the egg whites to egg yolk mixtures. Then return the egg yolk mixtures to the meringue and fold well.
Pour batter into the cupcake cases evenly and gently bang the muffin pan to release air bubbles.
Bake in preheated oven for about 25-30 minutes. The cakes will puff up and shrink back a little during baking.
Remove muffin pan from oven and drop 2-3 times on table top to release hot air and leave cakes to cool. Dust with some cocoa powder before serving.