In a medium saucepan, bring 1 cup of water and ½ cup of unsalted butter to a boil over medium heat. Once boiling, remove from heat and stir in 1 cup of all-purpose flour until a smooth dough forms. Return to low heat for 1-2 minutes, stirring constantly until the dough pulls away from the sides. Remove from heat, then carefully beat in 4 eggs one at a time until the mixture is glossy and pipable.

Preheat your oven to 400°F (200°C). Line baking sheets with parchment paper. Using a piping bag fitted with different size tips, pipe small mounds onto the sheets to create three sizes of cream puffs—these will form your tree layers. Bake for 25-30 minutes, or until the puffs are golden brown and firm to the touch. Remember to rotate the trays halfway through for even browning.

While the choux cools, prepare the pistachio cream filling. In a mixing bowl, whip 1 cup of heavy cream until soft peaks form. Gradually fold in ½ cup of pistachio paste and ½ cup of powdered sugar, mixing until the filling is thick and smooth.

Once your choux pastries are completely cooled, carefully slice the tops off each puff. Use a piping bag to generously fill each puff with the pistachio cream. Stack several filled puffs to form a tree shape, piping small swirls of cream in between layers for added stability. Dust the assembled tree lightly with powdered sugar to create a snowy effect just before serving.

To add festive flair, decorate your towering Christmas Tree Cream Puffs with optional shortbread star toppers or colorful sprinkles. You may also use green food color to enhance your whipped cream filling for a whimsical touch.