Place popcorn in a large bowl that has been sprayed with cooking spray; set aside. Line a baking sheet or work surface with waxed paper or foil. Stir sugar, cocoa, milk and vanilla together in a medium saucepan. Cook, stirring occasionally, until mixture registers 250º F on a candy thermometer. Stir in glycerin, if desired, and pour hot mixture over popcorn. Stir to coat popcorn completely. Spread popcorn onto prepared surface and allow to completely. Break into pieces to serve. Store in an airtight container. Note: popcorn will have more sheen with the addition of glycerin.