In a medium saucepan, combine 1 cup of water, 1/2 cup of butter, and 1/4 teaspoon of salt. Bring this mixture to a rolling boil over medium heat. Once boiling, remove the saucepan from the heat and add 1 1/4 cups of all-purpose flour all at once. Stir vigorously with a wooden spoon until the dough forms a ball and pulls away from the sides of the pan.
Let the dough cool slightly for about 5 minutes. Then, add the 4 large eggs, one at a time, mixing well after each addition until the dough is smooth and glossy. The dough should be thick but pipeable.
Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper. Using a piping bag fitted with a large round tip, pipe 1.5-inch rounds of dough onto the prepared baking sheet, leaving space between each puff.
Bake the choux pastry in the preheated oven for about 20-25 minutes, or until they are puffed and golden brown. Avoid opening the oven door during baking, as this can cause the puffs to collapse. Once done, remove from the oven and let them cool completely on a wire rack.
In a mixing bowl, whisk together the 3.4 oz of instant lemon pudding mix, 1 1/2 cups of heavy cream, 1 cup of milk, 1 teaspoon of vanilla, and the zest of 1 lemon. Mix until the pudding thickens and the mixture is smooth.
Once the choux puffs are cool, use a sharp knife to make a small slit in the side of each puff. This will allow steam to escape and make filling easier. Transfer the lemon cream filling into a piping bag and fill each pastry generously.
Dust the filled Lemon Cream Puffs with powdered sugar before serving, adding a final touch of sweetness. Arrange them on a serving platter and enjoy these delightful treats with friends and family!