Place the yolks, sugar, flour, and salt in a medium bowl and whisk to combine.
Heat the milk in a small pot until small bubbles form around the edge, and wisps of steam are rising from the surface.
Add the hot milk to the egg mixture, a little at a time, whisking to combine. When all the milk has been added, transfer the mixture back to the pot and place over medium low heat. Cook the pastry cream, whisking, until thickened. (It should register about 165 degrees F on a candy thermometer.)
Stir in the vanilla, and pour the pastry cream through a fine mesh strainer, into a heat-safe bowl. Use the back of a ladle to press the mixture through the sieve.
Press a layer of plastic wrap directly onto the surface, and refrigerate for at least 2 hours.
ECLAIR BASE
Preheat the oven to 425 degrees F.
Place the water, butter, sugar, and salt in a saucepan. Cook over medium high heat, until the butter is completely melted and the mixture is simmering.
Add in the flour all at once, and continue to cook, stirring, for about 5 minutes, or until the dough gathers itself into a ball and a film begins to form on the bottom of the pot.
Transfer the mixture to a mixing bowl, and beat on medium speed.
While continuing to beat, drop in the eggs, one at a time. Allow each egg to become fully incorporated before adding the next (about 30 seconds to a minute).
Transfer the mixture to a large piping bag fitted with a jumbo round tip. Pipe 4-inch long, 1-inch diameter lines onto parchment-lined baking sheets, allowing about 3 inches in between each.
Bake for 35 to 40 minutes, or until puffed, golden brown, hollow, dry, and light.
CHOCOLATE GLAZE
Heat the cream in a small pot until barely simmering.
Pour the hot cream over the chopped chocolate, and allow to stand for 5 minutes.
Whisk the chocolate and cream together until fully incorporated.
Whisk in the corn syrup and vanilla.
ASSEMBLY
Place the cooled pastry cream in a large piping bag fitted with a Bismarck tip.
Pierce the cooked eclair shells with the tip, and squeeze in the filling.
Dip the tops of the eclairs in the chocolate glaze.