Rinse pumpkin seeds thoroughly, removing all pumpkin strings. Pat dry with a clean towel.
Place seeds in a bowl and pour apple cider over them. Cover and soak for 2 hours or overnight in the fridge.
Drain seeds well after soaking, but do not rinse again.
Preheat oven to 325°F (160°C). Line a baking sheet with parchment paper.
In a bowl, toss seeds with brown sugar, cinnamon, salt, and melted butter if using.
Spread seeds in a single layer on the prepared baking sheet.
Roast for 35–40 minutes, stirring every 10 minutes, until golden brown and crisp.
Remove from oven and let cool completely before serving.
Store in an airtight container for up to 1 week.