Preheat oven to 375°F (190°C). Using a permanent marker, draw 9 (4½-inch-long) lines at least 2 inches apart on each of 2 sheets of parchment paper. Place, marker side down, on 2 separate rimmed baking sheets.
In a medium saucepan, bring butter, 1⁄2 cup (120 grams) water, milk, sugar, and salt to a rolling boil over medium-high heat. Add flour, and stir with a wooden spoon until combined. Cook, stirring constantly, until a skin forms on bottom of pan and mixture pulls away from sides of pan and forms a ball, leaving a thin layer of residue on bottom of pan, 1 to 2 minutes.
Transfer mixture to the bowl of a stand mixer fitted with the paddle attachment; beat at medium-low speed until dough cools slightly and is no longer steaming, 2 to 3 minutes. Add eggs, one at a time, beating until combined after each addition. (Batter will be shiny and thick.)
Transfer batter to a large pastry bag fitted with a large open star piping tip (Ateco #828). Holding bag at a 45-degree angle, slowly pipe along lines. (Piped batter should be 3⁄4 to 1 inch thick.) Using a wet finger, smooth ends if necessary.
Bake, one pan at a time, for 15 minutes. Rotate pan, and bake until fully puffed and deep golden brown, 5 to 10 minutes more. (Let second pan stand at room temperature until ready to bake.) Let cool completely on pans.
Using a skewer, poke 2 holes about 3⁄4 inch from ends on bottom of each éclair. Insert skewer into holes, and move it around to ensure cream will fill entire éclair.
Spoon Pastry Cream into a pastry bag fitted with a large open star piping tip (Ateco #828). Place piping tip into a hole, and apply gentle pressure to begin filling éclair. Pipe cream into second hole until éclair feels heavy. Repeat with remaining éclairs.
Holding an éclair by bottom half and parallel to surface of Ganache, dip top of éclair in Ganache. Still parallel, lift éclair out of Ganache. Then slowly lift one end of éclair, letting excess Ganache run off. Repeat with remaining éclairs. Let stand at room temperature or refrigerate until set. Best served same day as made.