1-Step 1: Prepare the Pastry Cream First, combine 1 cup whole milk, 1 cup heavy cream, ⅓ cup granulated sugar, the split vanilla bean (if using), and ¼ teaspoon salt in a saucepan. Heat this mixture until the sugar dissolves and it simmers, then remove from heat and let it cool for 10 minutes. In a separate bowl, whisk 5 large egg yolks and 3 tablespoons granulated sugar until the sugar starts to dissolve, then add 3 tablespoons cornstarch and whisk until fully combined. Next, temper the eggs by slowly whisking about ⅓ cup of the warm cream mixture into the egg mixture, followed by the remaining cream. Return everything to the saucepan and cook over medium heat, whisking frequently, until it thickens. Strain through a fine-mesh strainer, whisk in 4 tablespoons of unsalted butter pieces one at a time, and stir in 1 ½ teaspoons vanilla extract if you didn’t use the bean. Press plastic wrap onto the surface, cool for 30 minutes at room temperature, then refrigerate for 2-4 hours until chilled.

2-Step 2: Prepare the Choux Pastry Preheat your oven to 400°F (200°C) and line baking sheets with parchment paper. In a saucepan, bring 1 cup water, ½ cup unsalted butter (cut into pieces), and ¼ teaspoon salt to a boil. Add 1 cup all-purpose flour all at once, reduce heat to low, and stir constantly until the dough forms a ball and pulls away from the pan. Remove from heat, let cool slightly for about 5 minutes, then add 4 large eggs one at a time, beating vigorously after each until the dough is smooth and glossy. Transfer to a piping bag and pipe mounds about 2 inches wide and 1 inch tall onto the sheets, spacing them apart. Bake for 30 minutes until dry, golden brown, and hollow-sounding. Pierce each puff to release steam, return to the turned-off oven with the door cracked for 10 minutes, then cool on a wire rack.

3-Step 3: Fill and Finish Transfer the chilled pastry cream to a piping bag and pipe it into the cooled puffs through the hole. Just before serving, dust with powdered sugar. This whole process, including prep time of 1 hour, cook time of 35 minutes, and chilling time of 3 hours, totals about 4 hours and 35 minutes.