Instructions

1. Make the blackberry crème légère: In a small pot over medium heat, bring the milk and vanilla seeds and pod to a simmer. Turn off the heat, cover, and set aside to infuse, 30–40 minutes. 
2. In a medium bowl, vigorously whisk together the cornstarch, egg yolk, and ¼ cup plus 2 teaspoons of the sugar. Return the milk mixture to a simmer over low heat, then slowly pour half of the hot milk mixture into the yolk mixture, whisking continuously. Pour the yolk mixture into the pot, turn the heat to medium, and cook, using a silicone spatula to stir and scrape the bottom of the pot, until the pastry cream returns to a simmer. Continue cooking, stirring continuously, until the cream is thick enough to coat the back of the spatula, 2–4 minutes more. Remove from the heat and whisk in the butter. Scrape into a clean bowl, cover the surface with plastic wrap, and refrigerate until thoroughly chilled, at least 3 hours, or up to 24. 
3. In a food processor, blend the blackberries, lemon zest, remaining sugar, and a pinch of salt until the mixture is liquified but still pulpy. Transfer to a large bowl. 
4. In the bowl of a stand mixer, whip the heavy cream to soft peaks. Using a silicone spatula, fold 1¾ cups of  the whipped cream into the blackberry mixture. Transfer the remaining whipped cream to a small bowl, cover with plastic, and refrigerate. Clean and dry the bowl of the stand mixer.  
5. Remove and discard the vanilla pod from the chilled pastry cream, then whisk until smooth. Add a quarter of the blackberry whipped cream to the pastry cream and fold to combine, then gently fold the pastry cream mixture into the blackberry whipped cream until just incorporated. Cover with plastic and refrigerate until ready to use.  
6. Make the craquelin topping: Position racks in the upper and lower thirds of the oven and preheat to 375°F. In a medium bowl, stir together the flour, brown sugar, butter, and a pinch of salt to form a dough. Between two sheets of parchment, roll the dough to a thickness of ⅛ inch. (The shape isn’t important.) Freeze until firm but not solid, about 20 minutes. 
7. Meanwhile, make the cream puffs: In a medium pot over medium heat, bring the butter, sugar, salt, and ½ cup plus 2 teaspoons of water to a simmer, about 2 minutes. Turn the heat to low and use a large wooden spoon to mix in the flour until a smooth dough forms and cleans the sides of the pot, about 2 minutes more. (There should be no pockets of dry flour remaining.) Transfer the dough to the stand mixer fitted with the paddle attachment and set aside to cool for 10 minutes. 
8. Line two large baking sheets with parchment paper or silicone mats. Turn the mixer to medium speed and beat until the dough releases its steam and has cooled slightly, about 2 minutes. One at a time, beat in the eggs until fully incorporated and the mixture is smooth, about 1 minute after each addition. Continue to beat until the dough is elastic, 1–2 minutes more. Transfer the dough to a pastry bag fitted with a ½-inch piping tip. Onto the baking sheets, pipe the dough into 1-inch-diameter rounds with domed tops (If needed, wet a fingertip and softly press the tops into domes; if you have leftover pastry, bake the puffs in batches.) 
9. Transfer the craquelin dough to a work surface, remove the top piece of parchment, and press out rounds with a 1-inch-diameter cookie cutter. Drape the rounds on top of the choux pastry domes and sprinkle with pearl sugar. 
10. Bake, swapping and rotating the baking sheets halfway through cooking, until the puffs are golden brown with no pale spots, 17–20 minutes. Set aside on the baking sheets until completely cool. 
11. Using a paring knife, make a small slit at the bottom of each cream puff. Transfer the blackberry crème légère to a pastry bag fitted with a ¼-inch tip and pipe it into the cream puffs until completely filled. Transfer the reserved whipped cream to another pastry bag fitted with a star tip, and pipe about 2 teaspoons onto each cream puff. Top each puff with a halved blackberry, then dust with confectioners sugar and serve.