Pan di Spagna:

1. In a large bowl, or in the bowl of a stand mixer, add the eggs, the sugar, a pinch of salt, and the vanilla.
2. With a hand held mixer, or in the stand mixer fitted with the whisk attachment, whip the mixture for 15-20 minutes, or until light and fluffy, 4-5 times the volume that it was.
3. Preheat the oven at 180°C/350°F. I used convection bake at 170°C/340°F.
4. Sift the flour on a piece of parchment paper. Add it to the egg mixture with a spatula or whisk with movements that go from top to bottom, not circular, just enough to obtain a smooth mixture.
5. Pour in the prepared baking pan and do not tap the pan nor smooth the surface. Use an 18 or 20 cm (6" or 8") baking pan.
6. Bake in the preheated oven for about 30 minutes or until lightly golden and the top of the cake springs back when lightly touched with a finger.
7. Take the sponge cake out of the oven, cool slightly in the pan, then remove from the springform pan and turn upside down on a rack to cool completely.
8. Wrap well in plastic and refrigerate until the next day to cut and assemble.

Vanilla custard and chocolate custard:

1. Heat the milk in a saucepan, until almost boiling. Turn off heat.
2. In a large bowl, whisk the whole eggs with the sugar, vanilla, and flour, until you have a creamy texture without lumps.
3. Take a ladle of the hot milk and pour it over the egg mixture. Mix, then add another ladle, and mix again. Do this one more time. We are tempering the milk so that it does not curdle. Then, pour this mixture back into the saucepan with the rest of the milk.
4. Place the saucepan back on the stove over low/medium heat and cook, constantly stirring with a whisk, until the custard thickens.
5. Turn off the heat and put half of the custard in a bowl. Leave the rest in the saucepan and add the cocoa powder. Mix, and briefly cook on the stovetop on low until the chocolate custard is well blended. Turn off heat and put the chocolate custard in another bowl.
6. Cover the prepared custards with cling/plastic wrap and let them cool at room temperature. Then place in the fridge. I usually make them the day before and keep them in the fridge overnight.

Rum syrup:

1. Pour the water and sugar into a saucepan and cook over low heat for about 5 minutes, stirring occasionally. The sugar must dissolve and become a syrup.
2. Turn off the heat and let it cool. Then, add the Rum and stir.

Whipped cream:

1. In a large bowl, with a handheld mixer, or in the stand mixer fitted with the whisk attachment, beat the whipping cream on high for a minute or so, then slowly add the icing sugar. Beat until firm but do not overwhip.

Assemble:

1. Remove the film on both custards.
2. To the vanilla custard add two tablespoons of whipped cream and mix to blend (you've just made Chantilly cream).
3. With a long, serrated knife, cut the sponge cake into 3 layers.
4. You can use a cake ring to assemble the cake. I've simply assembled it on a 10-inch round disposable cake base.
5. Brush the first layer of sponge cake with the prepared rum syrup, making sure to soak it well. 
6. With an offset spatula, spread the chocolate custard on the first layer, leveling it well.
7. Cover with another disc of sponge cake. Pat it with a well saturated brush of rum syrup and then spread the vanilla custard/Chantilly cream.
8. Top with the last disc of sponge cake and again, pat it, making sure that the brush is saturated with rum syrup. Place the cake in the refrigerator to set a bit.
9. After, spread the top of the cake with whipped cream. Leave the sides naked or decorate with whipped cream, as you most like. I used a flower shaped piping tip to pipe the whipped cream onto the sides. I used the same flower shaped tip to decorate the top.
10. You can add some shaved chocolate, cocoa powder, sprinkles, or fresh fruit to the top.
11. Serve the cake immediately or keep in the fridge until ready to use.