Fill a medium pot with water and bring to a boil. Turn off heat, add chestnuts and let stand for 1 minute. Remove chestnuts from pot and set aside on paper towels to drain.
Using a serrated knife, carefully cut an X on the rounded side of each chestnut.
In a medium bowl, combine the chestnuts, sage leaves, butter and salt; toss to combine.
Line a baking sheet with foil. Pour the chestnut mixture onto the prepared baking sheet and arrange in a single layer. Gather edges of foil around the chestnuts, leaving a large opening on top.
Bake until the shells begin to peel back and the chestnuts are cooked through, 30-35 minutes.
Season chestnuts with flake salt, if using, and serve hot or warm.
Recipe by Two of a Kind at http://www.twoofakindcooks.com/roasted-chestnuts-with-sage-butter/