1. Preheat oven to 250ºF. Line 2 baking sheets with foil and mist with cooking spray.
2. In a small saucepan, combine butter, sugar, corn syrup and condensed milk over medium heat. Affix a candy thermometer to side of pan, taking care not to let it touch the bottom. Stir until sugar dissolves. Brush down sides of pan with a wet pastry brush and cook until mixture registers 238ºF, about 8 minutes. Remove from heat and stir in cinnamon, cocoa and salt. (Mixture will be thick.)
3. In a large bowl, pour cocoa-caramel mixture over popcorn, and stir to coat thoroughly. Divide mixture between 2 sheets and spread into a single layer. Bake popcorn for 30 minutes, stirring every 10 minutes, until coating has hardened. Let cool on sheets on wire racks, then break up any clumps and store at room temperature in an airtight container for up to 5 days.