In a medium saucepan, whisk together the egg, salt, sugar, and cornstarch until well combined.

Gradually add the milk, whisking continuously to avoid lumps.

Add the lemon peel and lemon juice, and place the saucepan over medium heat.

Cook the mixture, stirring constantly, until it thickens to a pudding-like consistency. This should take about 5-7 minutes.

Once thickened, remove the pudding from the heat and let it cool slightly. Set aside.

Preheat your oven to 180°C (350°F). Grease and flour a baking dish or line it with parchment paper.

In a large mixing bowl, beat the eggs, salt, and sugar together until light and fluffy.

Gradually add the vegetable oil and vanilla extract, mixing until well combined.

In a separate bowl, sift together the flour and baking powder. Gradually add the dry ingredients to the wet mixture, stirring until a smooth batter forms.

If the batter is too thick, add a small amount of milk to achieve the desired consistency. The batter should be pourable but not runny.

Pour half of the batter into the prepared baking dish, spreading it evenly.

Gently spoon the lemon pudding over the batter, spreading it out evenly.

Pour the remaining batter over the pudding layer, smoothing it with a spatula.

If using coconut flakes, sprinkle them evenly over the top of the batter.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.

Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Once cooled, slice the cake into squares or rectangles and serve. The cake can be enjoyed as is or with a dusting of powdered sugar on top.