15 cups popped popcorn (about 3/4 cup kernels)
Vegetable cooking spray
1 cup plus 2 Tbsp. firmly packed dark brown sugar
1/2 cup butter
1/2 cup dark corn syrup
1/4 teaspoon kosher salt
1 cup lightly salted dry-roasted peanuts
1 (10.5 oz.) package candy-coated peanut butter pieces (such as Reese's Pieces)
1. Preheat oven to 325°.
2. Spread popcorn in an even layer on a lightly greased (with cooking spray) heavy-duty aluminum foil-lined pan.
3. Stir together brown sugar and next 3 ingredients in a small saucepan over medium-low heat; bring to a simmer, and simmer, stirring constantly, 1 minute.
4. Pour over popcorn, and stir gently to coat.
5. Bake at 325° for 25 minutes, stirring every 5 minutes.
6. Add peanuts during last 5 minutes.
7. Remove from oven, and spread on lightly greased (with cooking spray) wax paper.
8. Cool completely (about 20 minutes).
9. Break apart large pieces, and stir in candy pieces.