Preheat oven to 375 degrees. Prepare 2 baking sheets with cooking spray, parchment paper, or silpat liners.

Over medium-high heat, combine butter, salt, sugar, and water. Bring to a boil. Stir in flour with a wooden spoon. Continue stirring, until a thin film forms on the bottom of the pan. (The spoon dragged through should leave a line.)

Remove from heat and transfer to a bowl. Allow to cool for 3 minutes. At eggs, one at a time. Stir very well after each addition. At first, the mixture will seem to separate and clump. Keep stirring! Once the mixture is smooth again, add the next egg and repeat until all 4 eggs are incorporated.

In a small bowl, whisk together remaining egg and 1 tbsp water.

Fill a pastry bag or ziploc bag with dough. Cut off the tip about 1/2 inch up, or fit with a 5/8 inch plain tip. Pipe 1-1.5 inch rounds onto the pans. Dip your finger in water and smooth the pointed peaks down.

Brush tops with egg wash.

Bake until golden brown, about 30 minutes. Let cool for 5-10 minutes and remove to a wire rack to cool completely.

Fill another pastry bag with prepared pastry cream, fitted with a small, sharp tip. Poke the tip into the side of the cream puff, and fill with pastry cream.

Serve immediately, or refrigerate for 1-2 days. You can also freeze for up to one month.

In a medium saucepan over medium high heat, add milk and heat until bubbles form on the side of the pan; right before boiling.

In a large bowl, whisk together 1/2 c sugar, egg yolks, and cornstarch.

As soon as the milk bubbles, very slowly drizzle the milk into the egg/sugar mixture while whisking constantly. Return the liquid to the saucepan.

Cook over medium high heat, stirring constantly, until the mixture comes to a boil and thickens.

Remove from heat and stir in vanilla and salt. Pour into a bowl, and cover in plastic wrap; pressing the plastic wrap onto the surface of the pastry cream.

When pastry cream is completely cool, combine the heavy whipping cream and 2 tbsp sugar in the bowl of a stand mixer or in a large bowl with a hand mixer, and whip until stiff peaks form. Fold whipped cream into pastry cream.