I don't know where this delicousness has been for my entire life or why I'd never heard of it until stumbling upon a recipe a few years ago. Shortly after that, we began hearing "choux pastry" and "creme pat" on the British Baking Show, and our new favorite dessert was born. This recipe is a mix of two that I found online, an eclair cake at allrecipes and Paul Hollywood's creme pat recipe.
ECLAIR CAKE
Preheat oven to 400 degrees. Grease a 9x13 baking dish.
Combine water, butter, and salt in a medium saucepan and bring to a boil over med high heat. Reduce heat to medium and stir in the flour. Cook and stir until the mixture pulls away from the sides of the pan and forms a ball (165-175 degrees F). Transfer the mixture to a mixing bowl, let cool for 2-3 minutes, and beat in the eggs, one at a time until fully incorporated. (I use a hand mixer, but this can be done by hand.) Spread the dough evenly in the bottom of the baking dish.
Bake in preheated oven until golden brown, 25-30 mins. The dough will rise, but then as it cools, will drop and form a boat shape. Cool completely in pan on wire rack.
Pour milk into a pan, bring barely to a boil, and remove from heat. Whisk sugar, egg yolks, and cornstarch together in a large bowl. Pour a little of the hot milk onto the egg mixture, whisking continuously (to avoid cooking the egg in chunks!). I use a hand mixer. In batches, whisk in the rest of the milk until well combined, and then return to pan.
Cook the mixture over gentle heat (med low), stirring continuously until thick. It will just come to a boil (watch for that first bubble!). Remove from heat; stir in butter and vanilla until well combined. Let cool down for quite a bit. If it's hot when poured into chilled choux pastry boat, it will compromise the pastry. Fill the pastry boat with as much creme pat as it will hold. You may have a bit leftover; you don't want it spilling outside the confines of the boat and over the sides. Refrigerate for an hour or two until well chilled before topping with ganache (below).
Place chocolate in a bowl. Bring cream almost to boiling in saucepan over medium heat. (I accidentally boiled it in the microwave, which seemed to have no ill effect.) Pour hot cream over chocolate and let soften for 1 minute. Whisk until smooth. Let mixture cool for 10 mins. or so to thicken, and then pour over cream filling, spreading to cover entire surface.
Chill at least two to three hours before serving.
If you have leftovers, go ahead and cover with plastic or foil. Will keep in fridge for a few days (as if!).