Combine milk, water, butter, sugar, and salt in a heavy-bottomed saucepan and bring to a full rolling boil.
Remove from heat briefly and add all flour at once, stirring vigorously with a stiff spatula.
Return to medium-high heat and cook for at least 2 minutes. Watch for the fond (a thin, floury film) to form on the bottom of the pan and the aroma to shift from raw flour to a toasty scent.
Emulsion
Transfer the panade to a stand mixer with a paddle attachment and beat on medium for 1–2 minutes to release steam.
Once the bowl is warm but comfortable to the touch (approx. 140°F), begin adding eggs in small increments.
Stop adding eggs once the dough achieves a satin sheen and passes the V-Test (falls slowly to form a smooth triangle) or the Trench Test (a finger-dragged path stays upright but the edges slightly slump).
Pipe and Bake
Lightly mist your parchment paper with water to create localized humidity.
Pipe 4-inch cylinders at a 90-degree angle; use a damp finger to tap down any sharp peaks.
Bake at 400°F for 15–20 minutes to trigger the steam rise. Do not open the oven door during this phase, or the shells will collapse.
Reduce heat to 350°F and bake for another 10–15 minutes until deep amber.
Exhaust: Use a toothpick to poke a small hole in the side of each shell. Return them to the turned-off oven for 5 minutes to dry the internal "webbing".
Glaze
Bring cream and corn syrup to a simmer; pour over chopped chocolate.
Let sit for 2 minutes, then whisk from the center until a glossy emulsion forms.
Dip the tops of fully cooled éclairs into the warm ganache.
Notes
Tactile Check: The shell must feel weightless, indicating the interior is hollow and dry.
Auditory Check: The bottom should sound hollow when tapped.
Commercial Stability: The shell should be bone-dry and crisp enough to support filling for 15 minutes without losing structural integrity.