Prepare the Eclairs
Preheat oven to 425.
Line baking sheets with parchment paper or a Silpat and set aside.
In a heavy bottomed saucepan, combine butter, milk, water, sugar and salt; bring to a boil.
Add the flour at once, reduce heat to medium-low and, using a wooden spoon, stir the mixture vigorously until the dough comes together.
Continue to stir for another minute, or until the dough is dry.
Transfer the dough to your mixer's bowl and let stand 2 minutes.
Add the eggs, one at a time, beating after each egg has been thoroughly incorporated into the dough.
Add the pumpkin spice and mix just until combined.
Using a pastry bag fitted with a large plain tip, pipe the dough onto the previously prepared baking sheets, leaving 2 inches of space between them. They should look like 6-inch, long hot dogs.
Bake for 12 minutes, then reduce the heat to 375, rotate the baking sheets, and bake until puffed up and light golden brown, about 8 to 10 minutes more.
Let cool on baking sheets.
In the meantime, prepare the Filling.
Combine egg yolks, pumpkin, sugar, flour, pumpkin pie spice and cornstarch in a mixing bowl; whisk until the mixture is completely combined and smooth. Set aside.
In a saucepan, warm the milk over low heat until hot, but not boiling.
Add half of the hot milk to the pumpkin mixture, whisking constantly.
Now take the pumpkin mixture and add it to the milk that's in the saucepan.
Continue to cook the mixture over low heat while constantly stirring, for 1 to 2 minutes, or until the custard is very thick.
Remove from heat and stir in the vanilla.
Cover with plastic wrap and place in the fridge until completely cooled; about 45 minutes.
In the meantime, prepare the Chocolate Ganache.
Place the chocolate in a medium mixing bowl; set aside.
In a small saucepan, cook the cream over medium heat just until it boils.
Remove from heat and pour the hot cream over the chocolate; whisk until melted and smooth. Set aside. You can also let it cool, cover and refrigerate until ready to use. Make sure to rewarm in a microwave when ready to use.
Assemble
Slice the eclairs horizontally, using a serrated knife and a gently sawing motion.
Using a pastry bag fitted with a medium-size plain tip, pipe the Pumpkin Cream into the eclairs, using just enough to fill the inside; about 1-1/2 tablespoons per eclair.
Dip the tops of the eclairs in the warm chocolate ganache and set on a sheet pan.
Chill, uncovered, for 1 hour.
Serve.