Spread popcorn in an even layer on a lightly greased (with cooking spray) sheet of heavy-duty aluminum foil lined 13" x 18" pan (a roasting pan works great).
Stir together brown sugar, butter, corn syrup, and salt in a small saucepan over medium-low heat, bring to a simmer, stirring constantly, for 1 minute.
Pour over popcorn and gently stir with a rubber spatula to coat.
Bake at 325° for 25 minutes, stirring every 5 minutes. Add peanuts during the last 5 minutes.
Remove from oven and spread on a lightly greased (with cooking spray) sheet of wax paper.
Cool completely for 20 minutes. Break apart into large pieces, and stir in Reese's. Store in an airtight container for up to 1 week.