Preheat your oven to 425 degrees F. This step is crucial as it ensures your pastry will be perfectly baked and crispy. While the oven heats up, line a baking sheet with parchment paper to keep the pastry from sticking.
In a medium saucepan, combine the mixed frozen berries, lemon juice, granulated sugar, and cornstarch. Stir everything together over medium heat. You’ll see the berries begin to break down and release their juices. If you want to add a hint of floral notes, toss in the optional ½ teaspoon dried culinary lavender flowers.
Bring the mixture to a boil, then reduce the heat to a simmer. Cook for about 5 to 6 minutes, stirring occasionally. You'll notice the liquid thickening as it simmers. Once thickened, remove it from the heat and let it cool completely, which will take about 30 minutes. This cooling step is important because it allows the mixture to set into a syrupy filling.
Once cooled, stir in the vanilla extract. You can even prepare this berry filling a day before for convenience! If you’re short on time, feel free to submerge the pot in an ice bath to cool it quicker.
Now, let's work with the puff pastry! Roll out one sheet of the puff pastry on the prepared baking sheet. Use a fork to pierce it all over, which prevents the pastry from puffing up too much.
Spread the cooled berry mixture evenly over the bottom sheet, leaving about a 1-inch border around the edges. This border is essential for sealing the pastry.
Brush the edges of the bottom pastry with a little water. This helps the top pastry stick better when you place it on top.
Carefully lay the second sheet of puff pastry over the berry filling, aligning it with the bottom sheet. Use a fork to press the edges together firmly, sealing in the delicious filling.
Using the fork again, poke a few holes in the top of the pastry. This will allow steam to escape while baking, ensuring a crisp finish. If any air bubbles form in the oven, don’t worry; they can be pierced easily after baking.
Brush the entire top with the egg wash (made from the large egg and water mixture), giving it a golden color when baked. Now it’s time to pop it in the oven!
Bake for 25 to 30 minutes, or until the top is golden brown and flaky. As it bakes, your kitchen will be filled with the most incredible aroma. Once baked, remove the pan and let it cool slightly before glazing.
While the pastry is cooling, prepare the lavender glaze. In a bowl, whisk together the powdered sugar, vanilla extract, lavender extract, heavy cream, and milk. If the glaze seems too thick, add milk one teaspoon at a time until it reaches your desired consistency. Feel free to add a drop of food coloring too for a whimsical touch.
Once the Giant Berry Pop Tart has cooled slightly, drizzle the glaze over the top. Add sprinkles if you like! Slice it up, serve it warm, and enjoy the delightful burst of flavors with every bite.