For the Choux Pastry:
½ cup cold water
4 tablespoons unsalted butter
1 pinch salt
½ cup all-purpose flour
2 large eggs, at room temperature
2 cups whole milk
1 tsp vanilla bean paste
6 egg yolks
2/3 cup sugar
1/4 cup cornstarch
1 tbsp unsalted butter, cold
4 oz semi-sweet chocolate, finely chopped (if making chocolate pastry cream)
5oz semi-sweet chocolate bars, finely chopped
1/2 cup heavy whipping cream, hot but not boiling
Optional: ½ tablespoon light corn syrup (for an extra glossy glaze)
Preheat the oven to 450 degrees. Line a baking sheet with a silicone mat or parchment paper.
Combine water, butter, and salt in a saucepan; bring to a simmer over medium heat. Add flour; stir with a wooden spoon until mixture comes together into a smooth dough and pulls away cleanly from the bottom of the pan. Transfer to a bowl and spread out with a whisk to speed up cooling. Cool until warm, about 10 minutes.
Whisk eggs, one at a time, into the pastry, mixing well after each addition. Finish mixing with a spatula. Scoop pastry into a pastry bag and pipe into 6 to 8 buns on the baking sheet. Smooth out tops with a wet finger.
Place buns in the preheated oven. Immediately reduce heat to 375 degrees and bake until browned and fully puffed, 30 to 35 minutes. Transfer buns onto a cooling rack and place rack back on the baking sheet. Place in the turned-off oven, leave door partially open, and let cool completely, at least 20 minutes.
Add the milk and vanilla paste to a medium saucepan, heat then place over medium heat and bring to a boil.
In a bowl, whisk the egg yolks and sugar until light and fluffy. Add the cornstarch and whisk vigorously until no lumps remain.
Whisk in 1/4 cup of the hot milk mixture until incorporated. Whisk in the remaining hot milk mixture, reserving the saucepan.
Pour the mixture through a strainer back into the saucepan. Cook over medium-high heat, whisking constantly, until thickened and slowly boiling.
Remove from the heat and stir in the butter. Let cool slightly. (Stir in your chocolate at this point if using)
Cover with plastic wrap, lightly pressing the plastic against the surface to prevent a skin from forming.
Chill at least 2 hours or until ready to serve.
Place the chopped chocolate in a large heatproof bowl, pour the hot whipping cream over the chocolate and let rest for 1-2 minutes. Stir until smooth and the chocolate has completely melted. Add the corn syrup and stir until combined. Let rest for about 5 minutes.
Cut cooled choux puffs in half using a serated knife. Using a piping bag, fill with pastry cream. Dip the tops of the filled puffs into the glaze and place them on a wire rack to let dry. Assemble and serve. Store leftover cream puffs in an airtight container in the fridge for up to 2 days, but best enjoyed the day of making.