1. Preheat the oven to 400°F. Line two baking sheets with parchment paper and set aside.

2. Put the butter, milk, sweetened condensed milk, ½ cup plus 2 tablespoons water, and salt in a saucepan and bring to a boil over medium-high heat, stirring occasionally. Add the flour and cook, stirring constantly with a heatproof spatula, scraping the bottom of the pan and folding the dough over and over, until it is smooth and just starts to stick to the bottom of the saucepan, about 7 minutes.

3. Transfer the dough to the bowl of an electric mixer fitted with the paddle attachment. Mix the dough on medium speed for 1 minute to allow the steam to escape and the dough to cool slightly. With the machine running, add 4 eggs and the yolk one at a time, mixing until well incorporated, about 2 minutes. Turn the mixer speed to high and mix for 10 seconds.

4. Transfer the dough to a pastry bag fitted with a ½-inch-diameter plain piping tip. Pipe out 1-inch-wide 1-inch-tall rounds 2 inches apart onto the prepared baking sheets; try to form a peak at the top of each. Lightly beat the remaining egg and brush on the tops of the puffs.

5. Bake for 10 minutes, then lower the temperature to 350°F and bake until the puffs are risen and golden brown, about 20 more minutes. Resist the temptation to peek in at the puffs as they bake or they may fall. Remove from the oven and cool completely on the pans set on a rack.

6. Meanwhile, pour the cream in the bowl of an electric mixer fitted with the whisk attachment and whisk until medium peaks form. (When you lift the whisk from the mixture, a peak will form and the very tip of the peak will fall back down.) Gently fold in the pastry cream until fully incorporated and refrigerate until ready to use.

7. When ready to serve, cut the puffs horizontally in half. Spoon or pipe 2 tablespoons of the pastry cream onto the bottom half of each puff, sandwich with the tops, and serve immediately.

Strawberry Cream Puffs: Fold ¼ cup strawberry jam into the whipped cream mixture.