Step 1

In a medium saucepan, heat the milk over medium heat until steaming (do not boil). Stir in the vanilla extract or vanilla bean paste.

Step 2

In a separate bowl, whisk together the sugar, cornstarch, salt, and egg yolks until smooth and pale.

Step 3

Slowly pour about ½ cup of the warm milk into the egg mixture, whisking constantly to temper the eggs.

Step 4

Pour the tempered mixture into the saucepan and cook over medium heat, whisking constantly, until thickened to a pudding-like consistency, 2 to 3 minutes. Remove from the heat and whisk in the butter and rum (if using) until fully incorporated.

Step 5

Transfer the pastry cream to a bowl, press plastic wrap directly onto the surface to prevent a skin from forming, and refrigerate until chilled, 2 to 3 hours.

Step 6

Preheat the oven to 400°F (200°C) and set an oven rack in the middle position. Line a baking sheet with parchment paper.

Step 7

In a medium saucepan over medium heat, combine the water, milk, butter, sugar, and salt. Bring to a boil, stirring occasionally, until the butter is fully melted. Reduce the heat to low, add the flour all at once, and stir vigorously with a wooden spoon until the mixture forms a smooth ball and pulls away from the sides of the pan, about 1 minute.

Step 8

Transfer the dough to the bowl of an electric mixer fitted with the paddle attachment. Let it cool for 5 minutes, then beat on low speed for about 30 seconds to release steam.

Step 9

With the mixer on low, add the eggs one at a time, beating well after each addition and scraping down the sides of the bowl as needed. Once all the eggs are incorporated, continue mixing for 1 minute more, until the dough is smooth, thick, and pipeable.

Step 10

Transfer the dough to a pastry bag fitted with a ½-inch round tip (at this point, you can hold the dough at room temperature for up to 2 hours). Pipe 1¾-inch mounds (they should be about 1 inch tall) onto the prepared baking sheet, spacing them about 1½ inches apart. Wet your finger and gently press down any pointed tips (don’t skip this step or they will burn).

Step 11

Bake for 22 to 25 minutes, without opening the oven until near the end of the baking time, until the puffs are golden and well-risen.

Step 12

If you're planning to pipe in the filling, remove the puffs from the oven and immediately use a paring knife or skewer to poke a ¼-inch hole in the bottom or side of each one—this helps release steam and keeps the insides light and crisp. Return them to the baking sheet and place them back in the oven.Turn the oven off, crack the door open slightly, and let the puffs sit inside for 30 minutes to dry out. (If you plan to slice the puffs in half instead of piping, skip the hole-poking step—just leave them in the oven with the heat off.)

Step 13

Before filling the cream puffs, whisk the chilled pastry cream until smooth to restore its creamy texture.

Step 14

You can fill the cream puffs using one of two methods: by piping the filling into a hole or by slicing them in half and adding the filling.For piped filling: If you poked a hole in each pastry, transfer the pastry cream to a piping bag fitted with a small round tip. Insert the tip into the hole at the bottom or side of each puff and gently pipe until filled.For sliced filling: Use a serrated knife to cut each puff in half horizontally. Pipe or spoon the pastry cream onto the bottom half, then place the top half back on.

Step 15

Dust with confectioners’ sugar and serve immediately.