Preheat and Prepare: Get your oven ready by preheating it to 180°C (350°F). Grease and flour a cake pan to ensure easy removal later.

Lemon Pudding Magic: In a saucepan, whisk together the egg, sugar, salt, cornstarch, lemon zest, and lemon juice. Gradually whisk in the milk, making sure there are no lumps. Place the pan over medium heat and cook, stirring constantly, until the mixture thickens significantly. It should reach a pudding-like consistency. Remove from heat and stir in the butter until melted. Set the pudding aside to cool slightly.

Whip Up the Cake Batter: In a large bowl, grab your eggs, sugar, and vanilla sugar. Using a hand mixer or whisk, beat them together until light and fluffy. This will take a few minutes, so be patient! Next, add the sour cream, vegetable oil, and lemon juice, mixing well to combine all the wet ingredients.

Dry Meets Wet: In a separate bowl, whisk together the flour and baking powder. Slowly add this dry mixture to the wet ingredients, mixing gently until just combined. Don't overmix!

Layer and Bake: Pour half of the cake batter into your prepared pan. Spread the cooled lemon pudding evenly over the first layer of batter. Gently pour the remaining cake batter on top, creating a beautiful two-toned layer.

Oven Time! Bake the cake in your preheated oven for 40 minutes. To check for doneness, insert a toothpick in the center. If it comes out clean, your cake is good to go!

Cool, Dust, and Enjoy! Let the cake cool in the pan for a few minutes before transferring it to a wire rack to cool completely. Once cool, dust the top with powdered sugar for an elegant finishing touch. Now, slice, serve, and savor the delightful combination of fluffy cake and melt-in-your-mouth lemon pudding!