Prepare the Lemon Filling - Grab a saucepan and add the granulated sugar, along with the cornstarch. Cook over medium heat and pour in around one cup of the boiling water. Mix continuously for a few minutes. At this point, both the cornstarch and sugar should dissolve. In a separate bowl, mix your egg yolks. Now pour in around 1/4 of the hot liquid with the yolks and mix well. Once mixed, pour the bowl into the saucepan (along with the rest of the liquid). Stir continuously and pour in the lemon juice and lemon zest. It should thicken in a few minutes. At this point, remove from heat and add the butter. Mix well and leave to one side to completely cool.

Make the Batter - Gather your cake ingredients and preheat your oven to 350F. Grab two cake pans and prepare them by greasing them with butter, and then flour them. Using a bowl, add the butter and mix until it becomes creamy. At this point, begin adding the sugar and continue mixing. Keep doing this until the mixture becomes light and fluffy. Now begin adding the egg yolks individually; only add the next egg once the previous egg has been mixed.

In a separate bowl, add the flour, salt, and baking powder. Alternately pour the milk and the flour mixture; stirring well throughout. Once combined, at the vanilla extract. In another bowl, beat egg whites until they reach stiff peaks. Fold in egg whites gently to the batter.

Bake the Cakes - Pour batter into your cake pans and place in the center of your preheated oven for around 23-28 minutes. You can check the cakes are ready by placing a toothpick into the center; if it comes out clean, they are baked. Remove from oven and leave to cool completely.

Make the Frosting - Beat the butter and cream cheese until combined. Now add the powdered sugar slowly; mixing well throughout. Finally, add the vanilla extract and stir. Leave to one side.

Assemble the Cake - Lay one cake onto a serving plate and add around 1/2 cup of frosting to a piping bag. Pipe a thick frosting ring around the edge, then add around one cup of the filling into the center of the ring. Spread with a spatula, or the back of a spoon. Place the second cake on top and repeat the steps to use up your frosting and filling. Now you can cover the top of the cake, along with the sides with the remainder of the frosting. You can now choose to garnish the cake with lemon wedges and coconut flakes if desired. Slice, serve, and enjoy!