Prepare the Custard: In a saucepan, whisk together the egg, granulated sugar, vanilla sugar (or extract), and cornstarch. Gradually add the milk while stirring continuously.
Cook the Custard: Place the saucepan over medium heat, stirring constantly until the mixture thickens into a smooth custard. Remove from heat and let it cool.
Make the Cake Batter: In a large mixing bowl, beat the eggs with a pinch of salt until slightly frothy. Add the granulated sugar and whisk until the mixture becomes pale and fluffy.
Incorporate Liquids: Pour in the milk and gradually add the vegetable oil while mixing.
Add Dry Ingredients: Gently fold in the ground almonds, all-purpose flour, and baking powder until you achieve a smooth batter.
Assemble the Cake: Pour half of the batter into a greased cake pan. Evenly spread the cooled vanilla custard over the batter, then top with the remaining cake batter.
Bake: Preheat the oven to 350°F (175°C) and bake for 25–30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Cool & Serve: Let the cake cool before slicing. Serve as is or with a light dusting of powdered sugar for an elegant touch.