Preheat oven to 400 degrees. Line a baking sheet with parchment paper, set aside.
Trim the top and sides of the brie with a sharp knife to remove the rind*.
Split the brie in half horizontally with the knife to form two halves, set aside.
Roll out the puff pastry on a lightly floured work surface just enough to remove the creases. Spread 2 tablespoons of blackberry preserves in the center of the pastry and top with 1 tablespoon of walnuts.
Place the bottom half of the brie on the pastry and top with 2 tablespoons of preserves, 1 tablespoon of walnuts, and 2 tablespoons of brown sugar
Top with the remaining brie, preserves, and walnuts.
Pull the puff pastry up and around the brie until completely covered. Place on the prepared baking sheet and brush with the egg wash.
Bake for 20 to 25 minutes until golden. Remove from the oven and allow to cool on the baking sheet for 5 minutes.
*Removing the rind is not a requirement, it is edible but it will hold the cheese in place. To achieve a super gooey cheese lava flow, you will need to remove most of it.
I twisted the excess dough into a knot on top of the brie hoping it would help hold the blackberries in place, but it actually did the opposite. Flat on top works better.
I used 2 tablespoons of preserves with 1/2 tablespoon of water to create a “glue” to hold the blackberries in place…but they still ended up rolling off.