Berry Filling:

1. Combine Ingredients: In a saucepan over medium heat, combine frozen berries, lemon juice, sugar, cornstarch, and culinary lavender flowers (if using).
2. Cook Filling: Bring to a boil, then reduce to a simmer and cook for 5-6 minutes, until thickened.
3. Cool Filling: Remove from heat and let cool completely (about 30 minutes). Stir in vanilla extract after cooling. You can also cool it in an ice bath to speed up the process or make it the day before.

Pastry:

1. Preheat Oven: Preheat oven to 425°F (220°C). Line a baking sheet with parchment paper.
2. Prepare Base: Place one sheet of puff pastry on the parchment paper and prick it all over with a fork.
3. Add Filling: Spread the cooled berry filling over the pastry, leaving a 1-inch border. Brush the border with water.
4. Top with Pastry: Carefully place the second sheet of puff pastry over the filling, aligning it with the bottom sheet.
5. Seal Edges: Use a fork to press the edges together to seal.
6. Vent Pastry: Prick a few holes in the top of the pastry with a fork to prevent bubbling.
7. Egg Wash: Brush the entire top with the egg wash.
8. Bake: Bake for 25-30 minutes, or until golden brown.
9. Cool: Remove from oven and let cool slightly before glazing.

Lavender Glaze:

1. Combine Ingredients: In a bowl, whisk together powdered sugar, vanilla extract, lavender extract, heavy cream, and milk until smooth.
2. Adjust Consistency: If the glaze is too thick, add more milk, 1 teaspoon at a time.
3. Add Color: Add a drop of food coloring (if using) and mix well.

Assembly:

1. Glaze: Drizzle the lavender glaze over the cooled pop tart.
2. Decorate: Top with sprinkles and culinary lavender flowers (if using).
3. Slice and Serve: Slice and serve warm or at room temperature.