Step 1: Preheat the Oven

1. Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan and line it with parchment paper for easy removal.

Step 2: Prepare the Cake Batter

1. In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.
2. Add the egg yolks one at a time, mixing well after each addition.
3. Stir in the fresh lemon juice, lemon zest, and vanilla extract.
4. Gradually add the flour and salt, alternating with the milk, until the batter is smooth.

Step 3: Whip the Egg Whites

1. In a separate bowl, beat the egg whites until stiff peaks form.
2. Gently fold the whipped egg whites into the cake batter using a spatula. This will give the cake its light and airy texture.

Step 4: Prepare the Custard Layer

1. In a medium saucepan, heat the milk over medium heat until warm but not boiling.
2. In a separate bowl, whisk together the sugar, egg yolks, and cornstarch until smooth.
3. Slowly pour the warm milk into the egg mixture, whisking constantly.
4. Return the mixture to the saucepan and cook over medium heat, stirring continuously, until it thickens into a custard.
5. Remove from heat and stir in the vanilla extract.

Step 5: Assemble and Bake

1. Pour the cake batter into the prepared pan.
2. Gently pour the custard over the cake batter. The custard will sink slightly, creating a layered effect.
3. Bake in the preheated oven for 40-45 minutes, or until the top is golden brown and a toothpick inserted into the cake comes out clean.

Step 6: Cool and Serve

1. Allow the cake to cool in the pan for 10-15 minutes before transferring it to a cooling rack.
2. Dust with powdered sugar and garnish with fresh berries or whipped cream before serving.