1Preheat oven to 325 degrees F (165 degrees C). Lightly grease a baking sheet.
2Place popcorn in a large bowl.
3Combine brown sugar, butter, corn syrup, and salt together in a saucepan; bring to a boil, stirring occasionally. Remove from heat and cool while stirring constantly for 2 minutes.
4Stir cinnamon, vanilla extract, and baking powder into butter mixture. Pour butter mixture over popcorn and toss to coat. Mix walnuts into popcorn mixture. Spread popcorn in a single layer onto the prepared baking sheet.
5Bake popcorn in the preheated oven until coating is set, 10 to 12 minutes. Cool slightly before breaking apart.