Creating this delightful dessert involves preparing both the cake and the custard filling. Follow these step-by-step instructions to ensure success.

1. Make the Cake:

1. Preheat the oven to 350°F (180°C) and grease a baking pan.
2. In a bowl, beat the egg and sugar until the mixture is smooth and creamy.
3. Add the oil and lemon zest, mixing well to incorporate.
4. Sift in the flour and baking powder, then stir in the milk to create a smooth batter.
5. Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.
6. Let the cake cool completely before assembling.

2. Prepare the Custard:

1. In a saucepan, combine the egg yolk, sugar, vanilla, and cornstarch. Mix until well combined.
2. Gradually add the milk while stirring over low heat. Continue stirring until the mixture thickens.
3. Once thickened, add the lemon juice and remove from heat to cool.

3. Assemble the Cake:

1. Once the cake is completely cool, spread the custard evenly on top.
2. Dust with powdered sugar before serving.