Preheat oven to 375°F. Line 2 rimmed baking sheets with parchment paper. Using a permanent marker, draw 10 (2 ½-inch-long) evenly spaced lines onto each parchment paper sheet. Turn parchment paper over.
In a medium saucepan, melt butter over medium heat. Add ½ cup water, milk, granulated sugar, and salt; bring to a boil. Using a wooden spoon, stir in flour. Cook, stirring constantly, until a skin forms on bottom of pan, 1 to 2 minutes. Transfer mixture to the bowl of a stand mixer fitted with the paddle attachment. Beat at low speed until dough is warm to the touch, approximately 1 minute. Add eggs and egg white, one at a time, beating after each addition until combined.
Transfer batter to a large piping bag fitted with a large round tip (Ateco #806) or cut with a ½-inch- wide opening in tip of bag. Holding tip approximately ¾-inch above parchment paper, apply even pressure to bag and slowly pipe out dough in an oblong shape along each drawn line. Toward the end of each line, begin lessening pressure on piping bag. Stop applying pressure, and lift bag, leaving a small curl at end of piped dough. Using a wet finger, smooth tips, if necessary. Repeat piping procedure with remaining drawn lines and baking sheet. Let stand at room temperature until ready to bake.
Bake one baking sheet at a time for 15 minutes. Rotate baking sheet, and bake until fully puffed and deep golden brown, 10 to 15 minutes more. Remove from oven and let éclairs cool completely on baking sheets on wire racks.
Using a ¼-inch round piping tip (Ateco #802), poke 2 holes approximately ¾ inch from each end on bottom of each éclair. Insert a wooden skewer into holes, and move it around to create a cavity for filling.
Place Pistachio Pastry Cream in a piping bag fitted with a ¼-inch round tip (Ateco #802). Place piping tip in one hole, and apply gentle pressure to begin filling éclair. Pipe Pistachio Cream into second hole until éclair feels heavy. Repeat with remaining Pistachio Cream and remaining éclairs.
In a small heatproof bowl, microwave candy melts in 30-second intervals until completely melted, stirring between each interval until smooth to create a glaze. Holding each éclair by its bottom half and parallel to surface of glaze, dip top of éclair into glaze. Slowly lift one end of éclair, letting excess glaze run off. Keeping éclair parallel to glaze, lift éclair out of glaze. Place on parchment paper, glaze side up. While glaze is still wet, garnish with chopped pistachios, if desired. Let glaze dry until firm before serving, 10 to 15 minutes.
Recipe by TeaTime Magazine at https://teatimemagazine.com/pistachio-eclairs/