Preheat oven to 425°F (220°C). Line 2 baking sheets with parchment paper, or silicone mat. In a medium saucepan with heavy bottom, combine water, butter, sugar and salt and bring the mixture to a boil over medium high heat.
4 tablespoons unsalted butter, 1 tablespoon granulated sugar, ¼ teaspoon coarse kosher salt, 2/3 cup all-purpose flour
Once the butter is fully melted and the mixture is boiling, remove from heat and add the flour all at once. Then put it back on the burner on medium heat.
Using a wooden spoon, continuously stir the mixture. The dough will start to form into a ball. Keep mixing for another 3-4 minutes until light crust forms on the bottom of the pan.
Transfer the dough into a medium bowl. Using a electric hand mixer, mix the dough for a few minutes on low speed until the mixture cools to room temperature. (Alternatively, you can use a wooden spoon. Mixer is not required here!)
Add the eggs one at a time, beating well after each addition. This will help to get egg absorbed into the dough quicker.
Transfer the batter into a pastry bag and pipe into 1.5-inch rounds couple inches apart on the prepared baking sheets. Level peaked tops with wet fingertip.
Bake for 25-30 minutes, rotating the sheets top to bottom and back to front half way. The pastry shells should be golden brown, crispy and puffed up. Transfer the shells onto a cooling rack to cool completely. (TIP: If the pastry shells fall flat when you remove from oven, it means they are under-baked. Simply put it back into the oven and bake for another 3-5 minutes, or until they puff up again.)