Prepare the Éclairs (can be done ahead of time and frozen):
- Preheat oven to 375°F. Line 2 baking sheets with parchment paper.
- In a medium sauce pan, melt the butter. Once the butter is melted, add water, salt, and sugar. Mix until combined and bring to slow boil.
- When it comes to a boil, reduce the heat and add in the flour, mixing constantly until it forms a soft, smooth paste/dough (Pâte à choux) and a film begins to cover the bottom of the pan (about 5-8 minutes).
- Remove from heat and let cool slightly, about 5 minutes. While cooling, set up an electric mixer fitted with the paddle attachment. Add mixture to the mixing bowl once cooled and mix on low to release additional heat.
- When it's cool enough to the touch, add eggs about ¼ cup at a time, combining completely and scraping the bowl as necessary. Dough should be well combined and sticky to the touch.
- Spoon dough into piping bag fitted with a large round tip. At a 45° angle with even pressure, pipe éclairs onto prepared baking sheets so they are about 4”-5” long and ¾” wide for regular size, or 2"-3” for minis with about 1-½” between each éclair.
- Bake for 20-30 minutes, until puffs have a golden brown color. Do not open the oven door prior to 20 minutes – this will let the steam out and interrupt the puff. When they have turned a golden brown, remove and let cool.
- Once cooled, using a wooden skewer, insert the skewer into one end of the éclair and move it around to create more space for filling.
Prepare the Crème Pâtissière:
- In a small pot, whisk together 1 cup of the milk, cream, maple syrup, and salt. Whisk over low-medium heat until it begins to steam and comes to a slight simmer.
- While the milk mixture is heating up, beat together the egg yolks and the remaining cup of milk. Sift together the cornstarch and flour gently over the egg yolk mixture. Whisk together well ensuring there are no lumps.
- When the milk mixture has come to a gentle simmer, remove from heat. While whisking the egg yolk mixture, slowly incorporate about one cup of the warm liquid. Return pot to low heat and slowly pour the egg mixture into the pot with the remaining milk mixture, continuously whisking.
- Warm over low-medium heat, continually whisking until the mixture begins to thicken and can coat the back of a spoon. Remove from heat, add butter, vanilla, and bourbon. Whisk until incorporated.
- Pour into a glass bowl and cover with plastic wrap, allowing the plastic wrap to lay directly on the filling to prevent a film from forming. Refrigerate until completely cool (at least 2 hours, preferably overnight).
Assemble the Éclairs:
- Pour the Bittersweet Chocolate Sauce into a bowl and warm slightly in the microwave.
- Place the crème pâtissière into a large piping bag fitted with a medium, circular tip (or a Bismarck tip). Pipe the filling into the eclairs using the hole created from the skewer. Fill until you can start to feel them puff, but not so much that the cream comes out of the cracks.
- Dip the filled eclairs into the chocolate sauce and set aside. When the chocolate sauce has had a chance to set, drizzle with maple syrup.
Serve slightly chilled.