In a small saucepan, whisk together 1 egg, 50 grams of sugar, and a pinch of salt until well combined.

Add 20 grams of cornstarch and slowly pour in 200 ml of milk, whisking continuously to avoid lumps.

Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and comes to a gentle boil.

Once thickened, remove from heat and stir in the zest of 1 lemon and the juice of ½ lemon.

Add 25 grams of butter and stir until melted and fully incorporated.

Transfer the custard to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and let it cool completely.

Preheat your oven to 180°C (350°F). Grease and flour a 9-inch (23 cm) round cake pan or line it with parchment paper.

In a large mixing bowl, beat 3 eggs, 150 grams of sugar, a pinch of salt, and 10 grams of vanilla sugar (or 1 teaspoon of vanilla extract) until the mixture is light and fluffy.

Gradually add 150 ml of vegetable oil while continuing to mix.

Stir in 50 grams of sour cream until well combined.

In a separate bowl, sift together 250 grams of all-purpose flour and 15 grams of baking powder.

Gradually add the dry ingredients to the wet mixture, folding gently to combine.

Finally, stir in the juice of ½ lemon.

Pour half of the cake batter into the prepared cake pan.

Carefully spoon the cooled lemon custard filling over the batter, spreading it evenly.

Pour the remaining cake batter over the custard, ensuring it covers the custard layer completely.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center of the cake comes out clean.

Remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Once the cake has cooled, dust the top with powdered sugar for a decorative touch.

If desired, sprinkle toasted almonds on top for added texture and flavor.

Slice and serve, enjoying the rich lemon flavor and creamy custard filling.