Freshly brewed espresso amps up the rich pastry cream that's piped into these delicate puffs. Martha made this recipe on episode 611 of Martha Bakes.
Prepare the pastry cream: Whisk together sugar, cornstarch, and salt in a medium saucepan. Whisk together milk, espresso, and egg yolks in a glass measuring cup. Add milk mixture to saucepan, along with butter. Cook over medium heat until mixture comes to a boil. Let boil 1 minute. Remove from heat and add vanilla.
Strain pastry cream through a fine-mesh sieve into a bowl. Cover with plastic wrap, pressing it directly onto the surface of the pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or up to 2 days.
Prepare the puffs: Preheat oven to 400 degrees. In a medium saucepan over medium-high heat, combine butter, sugar, salt, and water. Bring to a boil and immediately remove from heat. Using a wooden spoon, quickly stir in the flour until combined.
Return pan to medium-high heat and cook, stirring vigorously, until mixture pulls away from the sides and a film forms on the bottom of the pan, about 3 minutes.
Remove from heat and add eggs, one at a time, stirring vigorously after each addition, and waiting to add the next egg until the previous one is entirely incorporated.
Transfer dough to a pastry bag fitted with a 3/4-inch plain round tip (#809). On two parchment-lined baking sheets, pipe sixteen 2-inch rounds. Gently smooth the pointed peaks with a moistened finger, rounding tops to ensure even rising. Bake, rotating halfway, until puffed and deep golden-brown, 30 to 35 minutes. Transfer to a rack. Using a paring knife, make a small "X" in the bottom of each puff. Let cool completely.
Whisk the pastry cream until smooth and transfer to a pastry bag fitted with a 1/4-inch filling pastry tip (#230). Fill each puff with some of the pastry cream.
Make the glaze: Place the chocolate in a medium bowl. Bring the cream to a boil in a small saucepan over medium heat. Pour the hot cream over the chocolate; let stand 1 minute. Whisk until smooth. Dip the top of each cream puff in the chocolate and let stand at room temperature to set, about 20 minutes.