Preheat oven to 425°F (220°C). Line 2 baking sheets with parchment paper, or silicone mat.
In a medium saucepan, combine 2/3 cup (150ml) of water, butter, sugar and salt and bring the mixture to a boil over medium high heat. Once the butter is melted and mixture is boiling, add the flour all at once, reduce the heat to medium. 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 mixing bowl with paddle attachment and mix it for a few minutes on low speed until the mixture is at room temperature. Add the eggs one at a time, beating well after each addition. This will help to get egg absorbed into the dough quicker. The dough should not be too runny, yet not too thick. It should be just runny enough to fall back into a ribbon, as shown on the photo above.
Transfer the batter into a pastry bag and pipe into 1.5-inch rounds couple inches apart. Level peaked tops with a wet fingertip, if needed.
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. (Note: If the creme puffs go flat as they come out of the oven, it means they are slightly undercooked and needs to be bake a bit longer. Simply put them back in the oven and bake for a few more minutes.) Transfer the shells onto a cooling rack to cool completely.
To make the filling, whip the heavy cream until hard peaks form in a mixing bowl with whisk attachment. Fold in lemon curd and gently mix until smooth. Transfer the filling into a pastry bag with small round tip (I used Wilton #7).
When ready to serve, pipe the filling into each pastry shell through a small hole on the side. (Mine usually have a small hole somewhere, but you can also make a small slit with a knife.) Alternatively, cut the pastry shells horizontally and fill the shell that way. Arrange the creme puffs on a platter and dust with powdered sugar, if desired.
For step-by-step photos and additional notes, read the post above.*I used my homemade lemon curd. You can find the recipe here.