1-First, start by preparing the pastry cream filling. Whisk 4 large egg yolks with 3 tablespoons cornstarch until combined, then heat 2 cups whole milk and 1/2 cup granulated sugar in a saucepan until simmering. Gradually add the hot mixture to the egg yolks while whisking to temper them, strain it back into the saucepan, and heat while stirring constantly until it thickens to a custard-like consistency, reaching 185 190°F and forming large bubbles.

2-Remove from heat and stir in 1 tablespoon unsalted butter, 1 teaspoon pure vanilla extract, seeds from 1/2 vanilla bean (or an extra 1/2 teaspoon vanilla extract), and a small pinch of salt. Cover with plastic wrap directly on the surface and refrigerate for at least 3 hours or up to 24 hours. Next, make the choux pastry dough according to its recipe, which you can use right away or store in the fridge for up to 3 days.

3-Preheat your oven to 400°F (204°C) and line two baking sheets with parchment paper brushed with water to add humidity. Pipe the choux pastry into 4-inch logs, spacing them 3 inches apart, using a large round piping tip or a cut corner of a zip-top bag. Smooth any peaks with a moistened finger and brush with the egg wash made from 1 egg beaten with 1 tablespoon milk or water.

4-Bake at 400°F (204°C) for 20 minutes, then lower the temperature to 350°F (177°C) and bake for another 8 12 minutes without opening the oven door until golden brown. Let them cool completely on a rack before moving on.

5-For the chocolate ganache, heat 1/2 cup heavy cream until simmering, pour it over 4 ounces finely chopped semi-sweet chocolate, let it sit for 2-3 minutes, then stir until smooth and cool for 15 minutes until thickened. Fill the cooled shells by stirring the chilled pastry cream and piping it in through a small hole, or slice them open and spread the filling inside. Dip the tops in the ganache and let set for about 1 hour.