Preheat the oven to 350°F (175°C).

In a mixing bowl, cream the butter and sugar together until light and fluffy.

Add the eggs one at a time, mixing well after each addition.

Stir in the vanilla extract.

In a separate bowl, combine the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, alternating with milk, and mix until smooth.

Pour the batter into a greased cake pan and bake for 30 minutes or until a toothpick comes out clean.

In a saucepan, heat the milk and sugar over medium heat until warm.

In a separate bowl, whisk together the egg yolks and cornstarch until smooth.

Slowly pour the warm milk mixture into the egg yolk mixture, whisking continuously.

Return the mixture to the saucepan and cook over low heat, stirring constantly until thickened.

Remove from heat and stir in the vanilla extract.

Once the cake has cooled, slice it in half horizontally.

Spread a layer of custard on the bottom half of the cake.

Place the top half of the cake back on and cover the entire cake with the remaining custard.