Serves: 12 cream puffs
1) In a medium saucepan, combine the water, butter, sugar, and salt. Bring to a boil over medium heat. Remove the pan from the heat and sift the flour directly into the pan. Stir the mixture into a paste with a wooden spoon. Return the pan to a medium heat. Stir the mixture until it pulls away from the sides of the pan and is slightly shiny, about 3 minutes.
2) Transfer the paste to a medium bowl, and beat with a wooden spoon until slightly cooled, about 2 to 3 minutes. Add the eggs, one at a time, incorporating each one thoroughly before adding the next. Before adding the last egg, beat it in a small bowl. Adding it by tablespoons to the mixture, just until the batter is smooth and tight. If the batter starts to get loose, do not add all of the last egg.
3) Place a rack in the middle of the oven and preheat to 200C/Gas 6. Line a baking sheet with parchment paper.
4) Fill a large pastry bag, fitted with an open tip, with the cream puff mixture. Pipe twelve 6-cm circular mounds, 5-cm apart, onto the prepared pan. (Alternately, use a large spoon to form the dough into the mounds.) Dab the tops of each puff with a fingertip dipped in water to smooth the tops. Beat the egg yolk with a tablespoon of water and brush the surface of the puffs with the beaten yolk.
5) Bake in the middle of the oven for 15 minutes. Reduce the oven temperature to 180C/Gas 4; continue baking until golden brown and the puffs are light, airy, and crisp, about 30 to 35 minutes more. Cool on a rack.
6) Assemble the cream puffs. Cut the puffs in half with a serrated knife, and set the tops aside. (If the puffs are very moist inside, scoop out the excess dough and re-crisp them in a 180C/Gas 4. oven for about 5 minutes. Cool completely before continuing.) Put a heaping tablespoon of the fruit in the bottom half of the cream puffs. Pipe or spoon about 55ml of the whipped cream on top. Replace the puff tops. Refrigerate for up to 3 hours or serve immediately.
7) For the filling: Beat the cream in a medium bowl with an electric mixer or a whisk, until it begins to hold a loose peak. Sift the sugar into the cream. Continue beating until the cream holds a firm but still soft peak. Set aside.
8) Put the chocolate in a clean, medium, microwaveable bowl. Melt the chocolate in the microwave, at half power, at 30 second intervals, until softened and warm. Whisk until smooth and liquid.
9) Fold about 125ml of the whipped cream into the warm chocolate to lighten it. Add the lightened chocolate mixture back to the remaining whipped cream, folding until completely blended. Store in an airtight container until ready to use. It's best used the same day it is prepared.