In a medium saucepan, combine 1 cup of water and ½ cup of unsalted butter over medium heat. Bring to a boil and ensure the butter melts.

Reduce heat to low, quickly add 1 cup of all-purpose flour, and stir vigorously until a smooth ball forms (about 1-2 minutes).

Remove from heat and let cool for about 5 minutes.

Crack in 4 large eggs, one at a time, beating well after each addition until dough is shiny and smooth (about 3-5 minutes).

Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.

Pipe the dough into 2-inch rounds on the baking sheet. Bake for 25-30 minutes until puffy and golden brown, avoiding opening the oven door.

Cool baked puffs on a wire rack completely (about 30 minutes).

In a bowl, mix 1 cup of heavy cream, ¼ cup of sugar, and 1 teaspoon of vanilla extract. Whip until stiff peaks form (3-5 minutes).

Fill cooled cream puffs by cutting a small slit or using a pastry bag to inject the cream. Sprinkle with powdered sugar if desired.