If you love coconut, put these eclairs on your to-do list. The crisp, airy pastries are filled with coconut pastry cream and finished with a coconut glaze and toasted coconut.
Combine granulated sugar, cornstarch, salt, and toasted coconut in a medium saucepan. Combine coconut milk, whole milk, egg yolks, and vanilla in a large liquid-measuring cup. Place a fine-mesh sieve over a heatproof bowl. Add milk mixture to sugar mixture until fully combined and no lumps remain. Cook, stirring, over medium heat until mixture thickens and comes to a boil; let boil 2 minutes. Strain mixture through sieve into bowl. Cover with plastic wrap, pressing it directly onto surface to prevent skin from forming. Refrigerate until well chilled, at least 4 hours and up to 2 days.
Pate a Choux:
In a medium saucepan, heat butter, granulated sugar, salt, and 1 cup water over medium; bring to a boil. Remove from heat. Using a wooden spoon, quickly stir in flour. Cook over medium-high heat, stirring constantly, until mixture pulls away from sides of pan and a film forms on bottom, about 3 minutes.
Transfer to the bowl of an electric mixer fitted with the paddle attachment. Beat on low speed until slightly cooled, 1 1/2 to 2 minutes. With mixer on low, add eggs, one at a time, beating until each is fully incorporated and a soft peak forms when you touch dough with your finger.
Transfer to a pastry bag fitted with a 1/2-inch open star tip (such as Ateco #826). Using a pencil and ruler, make a template that will be the guide for piping out the eclairs: Draw a line crosswise on a sheet of parchment, about 1 inch from the edge of a short side. Draw a second line crosswise, 5 inches from the first. Draw a second set of lines on opposite end of parchment, then trace template to an additional sheet of parchment. Place templates, upside-down, on two rimmed baking sheets and use a small amount of dough to adhere templates to each corner of baking sheets.
Preheat oven to 400 degrees. Pipe dough between lines on parchment templates, holding pastry bag at a 45-degree angle and allowing dough to fall gently onto the surface. When you reach the end, stop squeezing pastry bag and slide the tip back along the surface of the eclair to release. Flatten tips of eclairs with a wet finger.
Bake 10 minutes, then reduce oven temperature to 350 degrees. Continue baking until evenly browned and firm to the touch, 20 to 30 minutes. Remove from oven and prick a small hole in bottom of each eclair to release steam. Transfer sheets to wire racks and let cool completely. (For extra-crispy shells, return cooled pastries to a 350-degree oven 20 minutes to dry thoroughly.)
Stir chilled pastry cream with a rubber spatula to loosen. Transfer to a piping bag fitted with a coupler and a filling tip (such as Wilton #230). Insert tip into bottom of each eclair; fill.
In a small bowl, whisk together confectioners' sugar and coconut cream. Using a spoon, carefully apply a line of glaze about 1 inch wide along top of each filled eclair, holding eclair vertically to allow excess to drip off the end. Smooth edges of glaze with your finger, if necessary. Garnish with toasted coconut, if desired. Glazed eclairs can be stored in refrigerator up to 1 day.