Over medium heat combine milk, whipping cream egg yolks, vanilla and sugar and cornstarch/water mixture. Cook, stirring with a whisk constantly, over medium heat until mixture thickens. Immediately cover with lid and place in refrigerator until cool.
Mix the French Vanilla cake mix as directed and pour it into 2 greased, floured 8 or 9 inch round cake pans.. Bake as directed. When the cakes are done, remove them from their pans and place each cake on a plate. Cut the rounded top of one cake off, so you have a flat surface.
Place the cake with the top cut off on your cake platter (it is the bottom layer) and drizzle it liberally with the cold coffee. Then pour enough of the cooled cream on top to completely cover it. Then place the sliced bananas on top of the cream, totally covering it.
Then take the other cake, turn it upside down and liberally drizzle it with the coffee. Invert it and place it on top of the cream and banana mixture. Then ice the cake with the espresso buttercream icing.
Mix the espresso powder into the vanilla until dissolved and set aside.
Using a whisk attachment on your mixer whip the butter on medium high for five minutes, stopping once to scrape the sides of the bowl. Reduce the speed to low and add the powdered sugar a little at a time, waiting until it's mostly incorporated before adding more. Once all the powdered sugar has been added, scrape the sides of the bowl and increase the speed to medium high and whip until fluffy; about a minute or two. Add the espresso and vanilla mixture and continue to mix on medium speed until completely incorporated. Add fine sea salt to taste.