In a medium saucepan, mix milk, half the sugar and salt. Cook until it comes to simmer.
In a medium mixing bowl, whisk together egg yolks, cornstarch and remaining sugar. Whisking constantly, slowly pour the milk mixture into egg yolk mixture. Cook over medium heat, until thickens. Whisk constantly.
Transfer to a mixing bowl of a stand mixer, add the butter and mix until butter is melted and the cream cools, about 5 minutes. Transfer to a smaller bowl and cover with seran wrap. Refrigerate for up to two days. Whisk before using. Mixture should be chilled before filling eclairs.
Make pate a choux:
Place butter, sugar, salt and water in a medium saucepan. Bring to boil. Remove from the heat.
Sift the flour over the mixture and stir with a wooden spoon until it pulls away from the sides and a thin film forms on the bottom of the saucepan.
Transfer the mixture into a large mixing bowl of a stand mixer and mix on low speed until the mixture cools to room temperature, about 1 minute.
In the meantime, preheat the oven to 450 degrees F. Line two baking sheets with parchment paper or silicone baking mats. Set aside.
Raise the speed of your mixer to medium and add eggs one at a time. Mix until soft peaks form. Transfer the dough into a pastry bag fitted with a large round tip or a Ziploc bag with a snipped corner (1" deep). Pipe 4 inch long and 1 inch wide pieces onto prepared baking sheets, leaving at least 2 inches of space between them (they will double or even triple in size).
Brush with egg wash. Bake eclairs for 18 to 20 minutes, until golden brown and cracks formed along the pastry. Cool on baking sheets for 3 minutes and transfer onto a wire rack to cool completely before filling and glazing.
For the glaze: microwave chocolate and coconut oil in a microwave-safe dish until melted. Stir till smooth.
Assembly: slice each eclair horizontally, fill with pastry cream and glaze.