Prepare the Sponge Cake: Preheat the oven to 180°C (350°F) and grease a cake pan. Beat egg yolks with sugar until creamy, then add lemon juice. In a separate bowl, beat egg whites with sugar until stiff peaks form. Sift flour, baking powder, and corn starch together. Gently fold them into the egg yolk mixture, alternating with the egg whites to maintain airiness. Pour the batter into the pan and bake for 25-30 minutes, or until a toothpick inserted comes out clean. Let the cake cool completely before assembling.
Prepare the Custard Filling: In a saucepan, whisk eggs, sugar, and vanilla essence. Add corn starch and mix well. Gradually pour in the milk while whisking continuously to prevent lumps. Cook over medium heat, stirring constantly, until the custard thickens. Remove from heat and allow it to cool completely.
Assemble the Cake: Slice the sponge cake into layers. Spread the custard filling evenly between the layers. Chill the cake for at least an hour before serving. Optionally, dust with powdered sugar or garnish with fruits.