Preheat your oven to 350°F (175°C) and butter a 9-inch round cake pan or use non-stick spray.

In a medium bowl, whisk together the flour, baking powder, and salt until well combined.

In a large mixing bowl, beat the softened butter and sugar until light and fluffy, about 3-5 minutes.

Mix in the eggs, one at a time, then add the vanilla extract until everything is combined.

Gradually mix in the flour mixture and milk, starting and ending with the flour. Stir in the lemon zest.

Pour the batter into the prepared cake pan and smooth the top. Bake for about 30-35 minutes or until a toothpick comes out clean.

While the cake is baking, combine egg yolks, sugar, milk, lemon juice, cornstarch, and salt in a saucepan over medium heat.

Stir constantly for about 8-10 minutes until the mixture thickens.

Once thickened, remove from heat and let it cool slightly.

When the cake is ready, let it cool in the pan for 10 minutes before transferring to a wire rack.

Once cool, slice the cake and drizzle the homemade custard on top.