Start by making the pastry cream. In a small bowl or cup, add 1⁄4 cup (60 grams) cold water and gelatin. Mix together, and set aside.
Pour milk and heavy cream into a medium saucepan, and warm over low heat until steaming, 5 to 6 minutes. (Do not bring to a boil.)
In a large bowl, combine egg yolks and sugar; whisk together until combined and slightly lightened in color. Add cornstarch and vanilla paste. Continue to whisk until combined. Slowly pour the warm milk mixture into the mixing bowl, adding a little at a time and whisking constantly to ensure that the eggs do not scramble. Once all the milk is added, pour the mixture back into the saucepan, and increase heat to medium- low. Bring to a boil, stirring constantly.
When the mixture begins to thicken, remove from heat, and add gelatin. Whisk together until the gelatin has dissolved. Add butter, and whisk until melted and combined. Pour through a fine-mesh sieve into a large bowl. Cover with plastic wrap, and refrigerate for at least 2 hours.
Once the cream has set in the refrigerator, divide the cream in half into two separate bowls. Stir lemon zest and lemon extract into one bowl.
Add freeze-dried blueberries to a food processor, and blend until it’s a fine powder. Add powder to a fine-mesh sieve to strain out large bits and then stir the powder into the other bowl of cream. Place both bowls back in the refrigerator until thick and cold.
Preheat 400°F (200°C). Roll Rough Puff Pastry to a 1⁄8-inch-thick rectangle (about 28×16 inches), or roll each sheet of puff pastry to 1⁄8-inch thickness. Cut pastry into 18 (41⁄2×21⁄2-inch) rectangles. Place 9 rectangles on each prepared pan. Top rectangles on each pan with another sheet of parchment paper. Place another baking sheet on top of parchment to weigh pastry down. Place another baking sheet on top to weigh them down.
Bake, one pan at a time, for 15 minutes. Allow the puff pastry squares to cool on a wire rack. Handle them with care; they’re very fragile!
While the puff pastry is cooling, add lemon and blueberry pastry creams to separate piping bags fitted with your desired piping tip.
Make the whipped cream topping by adding cold heavy cream and confectioners’ sugar to a mixing bowl and beat with an electric mixer on high speed until you reach stiff peaks. Add to a piping bag fitted with your desired piping tip.
Pipe the lemon and blueberry pastry creams onto 12 of the pastry rectangles, alternating between the two. Leave 6 of them without the cream.
Place one of the rectangles with cream on top of each other to make two layers. Then add the rectangle without the cream on top to create the third layer. Pipe the whipped cream on top and garnish with sliced blueberries and mint leaves.
