Preheat the oven to 425 degrees Fahrenheit. Line a baking sheet with parchment paper.

In a medium saucepan, combine the water, butter and salt. Cook until the butter completely melts.

Add the flour all at once and mix vigorously with a wooden spoon, until the dough comes together in a ball. Continue cooking for a minute or two. Take off the heat.

You can do the rest of the steps by hand, or using a stand mixer, with a paddle attachment. If you are mixing it by hand, cool the batter for 5 minutes. Add the eggs, one at a time, until they are fully incorporated. If you are mixing the batter in a stand mixer, place the hot batter into the bowl and mix with a paddle attachment, until you don't see any more steaming rising up from the bowl and then add the eggs one at a time. At first, the batter will look curdled, but it will eventually become smooth.

Pipe out the batter into 1 ½ inch circles, about 1- 1 ½ inches apart, on the prepared baking sheet. You can also drop the batter onto the cookie sheet with a spoon or use an ice cream scoop. When I pipe out the batter, I smooth out the top a bit with a damp finger.

Bake for 15 minutes.Without opening the oven, decrease the temperature to 375 degrees Fahrenheit, bake for another 8-10 minutes. While the cream puffs are baking, try not to open the oven door, or they will fall and deflate.You will have 35-40 cream puffs.

Cream puffs can be stored (without the pastry cream) in an airtight container for up to 2 days; freeze for longer storage. If you are not filling them right away, use a wooden skewer to poke some small holes in each cream puff, to let the steam escape. This will prevent the cream puffs from getting soggy on the inside.

Cut the cream puffs in half with a serrated knife, pipe the hazelnut pastry cream into the center and cover with the top of the cream puff.