For filling: In a large saucepan, heat milk and ¾ cup (150 grams) sugar over medium heat, whisking frequently, until sugar dissolves and mixture is steaming. (Do not boil.)
In a large bowl, whisk together egg yolks, cornstarch, salt, and remaining ¼ cup (50 grams) sugar. Gradually add warm milk mixture, whisking constantly. Pour egg yolk mixture into pan; cook over medium heat, whisking constantly, until thickened and bubbly, 4 to 5 minutes.
Strain mixture through a fine-mesh sieve into a medium heatproof bowl. Whisk in 2 tablespoons (28 grams) butter and vanilla bean paste or extract until butter is melted and mixture is smooth and well combined. Cover with plastic wrap, pressing wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until an instant-read thermometer registers 65°F (18°C) to 70°F (21°C), 2½ to 3 hours.
Preheat oven to 400°F (200°C). Line 2 rimmed baking sheets with parchment paper. Reserve 2 more baking sheets and 2 more sheets of parchment paper.
On a lightly floured surface, roll Rough Puff Pastry Dough into a 25×13-inch rectangle (about ⅛ inch thick). Using a pastry wheel, trim ½ inch off all sides of rectangle. Cut dough into 36 (4×2-inch) rectangles; place rectangles at least 1 inch apart on prepared pans. Place a sheet of parchment paper on top of dough rectangles; top parchment with another baking sheet.
Bake for 15 minutes. Carefully remove top pans and parchment, and bake until deep golden brown, 3 to 5 minutes more. Let cool completely on pans on wire racks.
In the bowl of a stand mixer fitted with the paddle attachment, beat remaining ½ cup (113 grams) butter at medium speed until smooth, about1 minute, stopping to scrape bottom and sides of bowl.
Whisk cold pastry cream until smooth. With mixer on medium speed, gradually add pastry cream to butter, beating until combined after each addition and stopping to scrape bottom and sides of bowl and paddle.
Transfer filling to a pastry bag fitted with a ¼-inch round piping tip (Ateco #802). Place 12 pieces of pastry on a baking sheet, and pipe 2 rows of large dots of filling on top of each pastry. Top each with another piece of pastry. Pipe 2 rows of dots of filling on top of each pastry. Spread a thin layer of filling on top of remaining pastry pieces; place on top of layered pastries.
For topping: In a large bowl, whisk together cold cream, sugar, and vanilla bean paste or extract until stiff peaks form. Transfer to a pastry bag fitted with a small petal piping tip (Ateco #102.)
Place 2 rows of blueberries on left side of each pastry and 1 row of strawberries on right side. Pipe whipped cream mixture between blueberries and strawberries. Refrigerate until ready to serve. Best served same day as assembled.
