Step 1: Preheat the Oven

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

Step 2: Prepare the Cake Batter

1. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
2. Beat in the eggs one at a time, mixing well after each addition. Add the lemon zest, lemon juice, and vanilla extract, and mix until combined.
3. In another bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
4. Pour the cake batter into the prepared cake pan and spread it evenly.

Step 3: Prepare the Custard Layer

1. In a medium saucepan over medium heat, combine the whole milk and sugar. Heat until the sugar dissolves and the mixture is warm, but do not boil.
2. In a separate bowl, whisk together the eggs, lemon juice, lemon zest, vanilla extract, and cornstarch until smooth.
3. Slowly pour the warm milk mixture into the egg mixture while whisking continuously to temper the eggs.
4. Carefully pour the custard mixture over the cake batter in the pan. Do not stir; the custard will settle to the bottom during baking.

Step 4: Bake

1. Bake in the preheated oven for 40–45 minutes, or until the top is lightly golden and the custard is set. A toothpick inserted into the center should come out clean.
2. Remove from the oven and allow to cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

Step 5: Serve

1. Once cooled, dust the top with powdered sugar if desired.
2. Slice and serve chilled or at room temperature.