Custard can be prepared a couple of hours ahead of time or a day before. I like to make it the day before because I can split the work in two days and custard gets a good amount of time to cool and set in the fridge.
Custard:
Vigorously whisk together the egg yolks and sugar until completely combined and lightened. Add flour and corn starch, whisk very well and set aside while heating the milk.
Heat milk, heavy cream, butter and salt until simmering.
Slowly add heated milk mixture into the egg yolk mixture stirring constantly. Whisk well, slowly. Return the mixture into the pot and bring to simmer stirring constantly. The mixture will thicken within a few minutes. (Keep whisking the custard while it's cooking, do not walk away or it will be nothing but lumps.) Take off heat! Whisk in the vanilla, set aside to cool to room temperature.
Transfer custard into a container, cover it with saran wrap or a lid and refrigerate for at least two hours.
Cake:
Preheat oven to 375 and line 17x11x1 pan with parchment paper and spray with cooking oil spray (make sure to get the sides too).
Beat eggs with electric mixer on high speed for 5 minutes until thick and light yellow.
Slowly beat in granulated sugar.
Lower speed to low and add water and vanilla extract.
Still on low, slowly add flour, baking powder and salt. Beat just until batter is smooth. Scrape sides and bottom of the bowl and mix just until combined.
Pour batter into prepared pan and spread evenly.
Bake for 8 minutes and touch the center of the cake to make sure it's done.
Run a spatula around the edges of the pan to loosen the cake. Let it rest for a couple of minutes.
Generously sprinkle a clean kitchen towel with powder sugar. Carefully, turn the cake pan upside down onto the towel. Gently remove parchment paper. Trim off crusty edges if necessary.
While the cake is still hot, gently roll cake with the towel from narrow end (don't make it too tight and don't press).
Cool on a wire rack for at least 30 minutes.
Gently unroll the cake. Spread cold custard all over the cake, leaving about an inch and a half one of the short ends(not the one you start rolling on). You might have a little bit custard left over.
Carefully roll the cake back up (starting with the side that has filling to the edge) and place cake on the serving dish, seam down.
Ganache:
Bring heavy cream to boil, be careful not to let it actually boil or it will raise.
Pour boiling cream over the chopped chocolate and stir until melted. Let it cool for a few minutes.
Pour ganache over the top of the cake roll and gently spread it around.