In a medium saucepan, heat 2 cups of milk over medium heat until just simmering. Remove from heat.

In a bowl, whisk together 4 egg yolks, 3/4 cup granulated sugar, 1/4 cup cornstarch, and 1 tablespoon lemon zest until smooth.

Gradually whisk in the heated milk until combined. Return to the saucepan over medium heat, and continuously stir until thick, about 5-7 minutes.

Remove from heat, stir in 1/2 cup lemon juice, and allow it to cool.

Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a large mixing bowl, beat 4 eggs and 1 cup granulated sugar until light and fluffy (about 5 minutes).

Gradually add 1 cup of flour, 1 teaspoon baking powder, and 1/2 teaspoon salt. Mix until well combined.

Pour the batter into the prepared pan and bake for 25-30 minutes, or until a toothpick comes out clean. Allow cooling.

Once the sponge cake is cooled, cut it in half horizontally.

Spoon a generous layer of the lemon custard between the two layers of cake, then spread the remaining custard on top.

Dust with powdered sugar before serving.