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

Cream the softened butter and granulated sugar in a mixing bowl until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, mixing well after each addition. Stir in the lemon zest, lemon juice, and vanilla extract until combined.

Whisk together the flour, baking powder, and salt in another bowl. Gradually add to the wet ingredients, mixing until just combined.

Pour the cake batter into the prepared cake pan and spread it evenly.

Combine the whole milk and sugar in a saucepan over medium heat until the sugar dissolves, about 5 minutes.

Whisk the eggs, lemon juice, lemon zest, vanilla extract, and cornstarch in a separate bowl until smooth.

Slowly pour the warm milk mixture into the egg mixture while whisking continuously to prevent curdling.

Carefully pour the custard mixture over the cake batter without stirring.

Bake in the preheated oven for 40-45 minutes, until the top is lightly golden and the custard is set.

Cool the cake in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

Dust the top with powdered sugar as desired and serve.

Enjoy the delightful flavor of your homemade Lemon Custard Cake!