Choux Pastry Dough
Put the water, butter, and salt in a medium saucepan and bring to a boil over high heat.
Add the flour and stir with a wooden spoon until the ingredients come together into a smooth, uniform dough.
Transfer the mixture to the bowl of a mixer fitted with a paddle attachment. Start paddling on low speed, then add eggs, one at a time, until thoroughly absorbed, mixing for one minute between eggs. Finish with the final egg and mix for an additional 2 minutes. The dough will be very thick and sticky.
Use the dough immediately. It does not refrigerate well.
Preheat oven to 450 degrees.
Transfer choux dough into a pastry bag fitted with a #6 plain tip (I used a # star tip instead; you could also use a large ziploc bag with the tip snipped off). The dough should be piped (1 inch wide and 3 inches long*) two inches apart onto parchment paper lined baking sheets.
Bake the eclairs in the oven, in batches if necessary, until they are golden brown and have set enough that you can pick them up easily off the baking sheet; 15 - 20 minutes.
Remove the tray from the oven and let the eclairs cool on the baking sheet for about 20 minutes, you don't need to transfer them to a rack to cool.
Use a serrated knife to slice through the eclairs.
Fill a pastry bag with the custard and pipe the filling into the eclairs (you can use the same bag you used for piping the dough, just rinse it out and dry it first).
Ice the eclairs following the recipe below.
To ice the eclairs, spoon the heaping tablespoons of nutella into a small, microwaveable bowl with the water. Microwave for about a minute to melt the nutella slightly.
Whisk in the sugar until the mixture is uniformly smooth.
Use a spoon or small spatula to use the tops of the eclairs.
Serve the eclairs right away.
They can also be refrigerated in an airtight container for up to 2 days or frozen in the freezer for about a month. Eclairs can be eaten straight from the freezer as well if you wish (makes them kind of taste like they are filled with ice cream).