Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.

In a medium saucepan, combine the water and butter. Heat over medium heat until the butter melts and the mixture comes to a boil.

Reduce the heat to low and add the flour and salt all at once. Stir vigorously with a wooden spoon until the mixture forms a ball and pulls away from the sides of the pan, about 1-2 minutes.

Remove from heat and let cool for 5 minutes. Add the eggs one at a time, mixing well after each addition until the dough is smooth and glossy.

Transfer the dough to a piping bag fitted with a large round tip. Pipe 1-inch (2.5 cm) rounds onto the prepared baking sheet, spacing them about 2 inches (5 cm) apart.

Bake for 20-25 minutes, or until the profiteroles are puffed and golden brown. Do not open the oven door during baking. Let them cool completely on a wire rack.

In a medium saucepan, combine the milk, sugar, cocoa powder, cornstarch, and salt. Whisk until smooth and bring to a gentle simmer over medium heat.

In a separate bowl, whisk together the egg yolks. Slowly pour a small amount of the hot milk mixture into the yolks, whisking constantly to temper the eggs.

Pour the egg mixture back into the saucepan and cook over medium heat, whisking constantly until the mixture thickens and bubbles, about 2-3 minutes.

Remove from heat and stir in the chopped dark chocolate and vanilla extract until smooth. Transfer the pastry cream to a bowl, cover with plastic wrap directly on the surface, and refrigerate until chilled.

In a small saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and add the chopped semisweet chocolate. Let it sit for a minute, then stir until smooth.

Once the profiteroles are cool, use a small knife to make a slit in the side of each one. Fill a piping bag with the chilled chocolate pastry cream and pipe it into each profiterole until filled.

Dip the tops of the filled profiteroles into the chocolate glaze, allowing any excess to drip off. Place them on a serving platter and let the glaze set for a few minutes.