Mix Ingredients: In a saucepan, whisk together the egg, sugar, salt, and starch. Stir in the lemon zest, lemon juice, and milk.

Cook: Cook over medium heat, stirring constantly, until the mixture thickens.

Add Butter: Once thickened, remove from heat and stir in the butter until smooth. Set aside to cool.

Preheat Oven: Preheat your oven to 180°C (356°F).

Beat Eggs and Sugar: In a large bowl, beat the eggs with the sugar and vanilla sugar until light and fluffy.

Mix Wet Ingredients: Mix in the sour cream, vegetable oil, and lemon juice until well combined.

Fold in Dry Ingredients: Sift in the flour and baking powder, and gently fold until the batter is smooth.

Prepare Pan: Grease a cake pan and pour the batter into the pan.

Bake: Bake in the preheated oven for about 40 minutes, or until a toothpick inserted into the center comes out clean.

Cool: Let the cake cool in the pan for a few minutes, then turn out onto a wire rack to cool completely.

Slice Cake (Optional): Once the cake has cooled, slice the cake horizontally to create layers.

Spread Pudding: Spread the lemon pudding between the layers and over the top of the cake.

Finish: Sprinkle powdered sugar over the top for a sweet finish.