1. Cook the Pumpkin Pastry Cream: Whisk brown sugar, cornstarch, cinnamon, pumpkin pie spice, and salt in a saucepan. Gradually add heavy cream and egg yolks. Cook over medium heat, whisking constantly until thick and bubbling, then continue for 1 minute, lowering the heat if needed.

2. Add the Pumpkin and Chill: Remove from heat and stir in pumpkin purée, butter, and vanilla until smooth. Transfer to a shallow container, cover directly with plastic wrap, and refrigerate for at least 2 hours, until completely cold.

3. Cook the Choux Dough: Preheat the oven to 425°F (220°C) and line a baking sheet with parchment. Bring water, milk, butter, salt, and sugar to a boil. Remove from heat, stir in the flour, then cook for 1–2 minutes, stirring until the dough forms a ball and leaves a thin film on the pan.

4. Add the Eggs Gradually: Transfer the dough to a bowl and cool for 5–10 minutes, until warm (below 125°F/52°C). Beat in 3 eggs one at a time, then gradually add the beaten fourth egg until the dough is glossy and forms a thick V when lifted. You may not need all of it.

5. Portion and Bake: Pipe or scoop 12 mounds, about 2 inches wide and 3 inches apart. Smooth the tips with a damp finger. Bake at 425°F (220°C) for 15 minutes, then reduce to 350°F (175°C) without opening the oven. Bake another 20–30 minutes, until golden brown and firm.

6. Vent and Cool: Cut a small slit in each puff and return to a 300°F (150°C) oven for 5 minutes to release moisture. Transfer to a rack, split horizontally when cool enough to handle, and let cool completely. The centers should be hollow and dry.

7. Fill and Finish: Whisk the cold pumpkin cream until smooth, then pipe or spoon into the cooled shells. Replace the tops and dust with powdered sugar before serving. Keep refrigerated and enjoy the same day for the best texture.