Sauté 1 1/3 cups finely chopped shiitake mushrooms in 1 tablespoon butter until tender.
Roll 1 pastry sheet into a 13- x 11-inch rectangle. Stir together butter, and salt. Brush half of butter mixture, and spread half of the mushroom mixture over pastry sheet.
Roll up pastry, jellyroll fashion, starting with each short side and ending at middle of pastry sheet. Repeat procedure with remaining pastry sheet, butter mixture and mushroom mixture.
Cut rolls crosswise into 1/3-inch-thick slices. Place on lightly greased baking sheets. Whisk together egg yolk and 1 tablespoon water, and brush evenly over pastries.
Bake at 375º for 10 to 15 minutes or until golden brown.
Top each pastry with a 1/2-inch cube of Brie in last 3 minutes of baking.