Whisk the whole milk, granulated sugar, and cornstarch for the filling in a saucepan until smooth. Cook over medium heat, whisking constantly, until thick and bubbling, then remove from the heat and stir in the butter, vanilla extract, and salt. Cover the surface and refrigerate until cold.
Preheat the oven to 425°F and line a large baking sheet with parchment paper. Bring the water, whole milk, butter, sugar, and salt to a full simmer in a medium saucepan.
Add the all-purpose flour and cornstarch at once and stir vigorously over medium-low heat for about 2 minutes, until the dough forms a smooth ball and a thin film develops on the bottom of the pan. Transfer the dough to a mixing bowl and cool for 5 minutes.
Beat the aquafaba into the warm dough in 3 additions, mixing thoroughly after each addition, until the pastry is smooth, glossy, and soft enough to pipe while still holding its shape. Beat in the baking powder last.
Pipe the dough into twelve 4-inch logs on the prepared baking sheet. Bake for 15 minutes at 425°F, reduce the oven to 350°F without opening the door, and bake for another 22 to 26 minutes until deeply golden and crisp.
Turn off the oven, make a small opening at each end of every eclair, and leave them in the warm oven for 10 minutes to dry. Transfer them to a rack and cool completely before piping the chilled vanilla filling into the centers.
Heat the heavy cream until steaming and pour it over the chopped chocolate. Let stand for 2 minutes, stir until smooth, then dip the tops of the filled eclairs into the glaze and allow the chocolate to set.
