Preheat oven to 375°F (190°C). Using a permanent marker, draw 3 lines each lengthwise 4 inches apart on 2 sheets of parchment paper (6 lines total). Turn over, and place on 2 rimmed baking sheets.

In a medium saucepan, bring butter, ½ cup (120 grams) water, milk, sugar, and salt to a rolling boil over medium-high heat. Using a wooden spoon, stir in flour. Cook, stirring constantly, until a skin forms on bottom of pan and mixture pulls away from sides of pan and forms a ball, about 1 minute. Transfer mixture to the bowl of a stand mixer fitted with the paddle attachment. Beat at low speed until dough cools slightly, about 30 seconds. Add eggs, one at a time, beating until combined after each addition. (Batter should be shiny and slowly move back together when a spatula is dragged through it; dough should also fall off mixer paddle so excess is left hanging in a “V” shape.)

Transfer batter to a large pastry bag fitted with a ½-inch round piping tip (Ateco #807). Place one long side of a prepared pan closest to you. Holding pastry bag at a 45-degree angle, slowly pipe batter in straight lines from top drawn line to center drawn line, spacing piped lines 1 inch apart. (Éclairs should be 1 inch wide.) Use a wet finger to smooth ends, if necessary. Starting from spaces left in first row of éclairs, repeat process, piping from center drawn line to bottom drawn line. Repeat procedure, piping between lines on remaining prepared pan. Let stand at room temperature until ready to bake.

Bake, one batch at a time, for 10 minutes. Rotate pan, and bake until fully puffed and deep golden brown, about 15 minutes more. (If not fully baked, they will deflate as they cool.) Let cool completely on pans.

Using a ¼-inch round piping tip (Ateco #802), poke a hole in center of top of each éclair. Insert a wooden skewer into hole, and move it around each side to ensure curd will fill entire éclair.

Place Lemon Curd in a pastry bag fitted with a ¼-inch round piping tip (Ateco #802). Place piping tip in hole, and apply gentle pressure to begin filling one side of an éclair; pipe into other side of éclair until it feels heavy. Repeat with remaining Lemon Curd and remaining éclairs. (Each éclair will have about 35 grams of Lemon Curd.)

Spoon Swiss Meringue into a pastry bag fitted with a ½-inch French star piping tip (Ateco #864). Pipe swirls onto éclairs. Using a handheld kitchen torch, carefully brown meringue. Best served same day.