1
Preheat oven to 350°F and line a baking sheet with parchment paper. Set aside.
2For the choux pastry: In a large pot, combine water, butter, salt, and sugar. Heat on medium-high heat until the butter is completely melted.
3
When the butter is completely melted, add in the flour (all at once) and stir with a wooden spoon until a dough forms and pulls away from the sides and bottom of the pot. It will turn into a smooth dough ball.
4
Continue cooking the dough, moving it around, and stretching it with your spoon for another 2 minutes, this will ensure the flour is slightly cooked and any extra moisture is released. A thin film will form on the bottom of the pot. Remove from heat after 2 minutes.
5
Transfer the dough into a large mixing bowl and allow to cool to room temperature. Mix the dough every couple of minutes so it cools faster.
6
Once the dough has cooled, use a stand mixer to incorporate the eggs, one at a time, into the dough.
7
Add in one egg, then beat until the egg is incorporated. Repeat this step with all eggs. You should end up with a sticky and thick dough that is able to be piped and forms a thick ribbon when pulled up with a spatula. Depending on the size of your eggs, you may need 4 or 5 to achieve the proper texture. A note of caution: It will not look good when you start adding in the eggs. In fact, it will look like you did something wrong. However, the mixture just needs some whisking until it turns into a sticky and thick dough.
8
Transfer the pastry dough into a piping bag fitted with a star tip. Pipe as many puffs as you'd like, but keep them all the same size so they cook evenly. Pipe the dough in a circle and work your way up, but avoid making a peak as it will burn in the oven. If you end up with peaks, just flatten them down slightly.
9
Bake the puffs for 30 minutes, and do not open the oven during the baking time. After 30 minutes, the puffs should be golden brown. If not, cook for another 5-10 minutes. Remove from the oven and cool completely. A note of caution: Do not open the oven while the puffs are baking. If you open the oven door, the pastry will not puff up.
10For the custard: Add in half of the milk to a medium pot and reserve the other half. To the pot with the milk add in the granulated sugar, vanillin, heavy cream, and vanilla extract. Heat on medium heat until just starting to boil.
11
While the milk heats up, to a mixing bowl add in the reserved milk, egg yolks, and flour. Whisk very well until the flour is combined and there aren't any lumps.
12
When the milk begins to just boil, add in the milk and flour mixture and whisk quickly. Continue whisking and cooking the custard until it becomes very thick. It will be slightly thicker than pudding. Once it has become very thick, continue cooking it for another 2 minutes. Remove from heat.
13
Transfer the custard to a bowl and cover with plastic wrap, making sure the plastic wrap touches the top of the custard (so a skin doesn't form). Cool to room temperature and then in the refrigerator until cold. Transfer to a piping bag when cold.
14To assemble: Cut each puff horizontally, pipe in the vanilla custard on the bottom piece, and top with the top piece. Dust with powdered sugar before serving. Store leftover assembled cream puffs in the refrigerator covered with plastic wrap.