Preheat the oven to 425 degrees F. Line a large baking sheet with parchment paper and set aside.

In a medium heavy saucepan, bring the milk, butter, sugar and salt to a boil over high heat. Once the butter has melted, add the flour all at once stirring to combine.. Reduce the heat to medium-low and stir briskly with a heavy wooden spoon until the paste is smooth and pulls away in a ball from the sides of the pan, 1 to 2 minutes.

Remove from the heat and let cool slightly, 1 to 2 minutes. Stirring constantly with a heavy wooden spoon, add 2 of the eggs 1 at a time, beating well after the addition of each. Mix until smooth. (Alternately, beat in the bowl of a mixer.) Let the dough cool.

In a small bowl, beat the remaining egg with 1 teaspoon of water to make an egg wash.

Transfer the dough to a pastry bag fitted with a plain tip and pipe out 16 ping pong ball-sized rounds onto the prepared sheet, about 2 inches apart. (Alternately, spoon onto the baking sheet with a large spoon.) Lightly brush egg wash on the tops and sides of the dough and bake for 15 minutes. Reduce the temperature to 375 degrees F and bake until the balls are firm to the touch and hollow inside, about an additional 8 to 10 minutes. Remove from the oven and let cool completely.

To make the Mocha Pastry Cream, add the semisweet chocolate pieces and espresso powder to the milk-egg yolk mixture in the Vanilla Pastry Cream recipe while it is thickening on the heat, before straining. Let sit for 2 minutes, then stir to melt the chocolate and incorporate the coffee. Strain into a clean container and cool completely in the refrigerator before using.

Fill a pastry bag with pastry cream. With the end of a wooden spoon, poke a hole into the bottom of each pastry round. Insert the tip of the pastry bag into the hole of each pastry and fill with the cream.

Arrange 4 profiteroles on each of 4 dessert plates. Drizzle with the chocolate sauce, top with the nuts, and sprinkle with the sugar. Serve immediately.