Preheat the oven to 250 degrees Fahrenheit and line a baking sheet with parchment paper or foil. You may need 2 baking sheets based on the size that you are using.
In a large saucepan, heat the butter until melted. Then stir in the sugar, corn syrup, salt and cocoa powder.
Bring the mixture to a boil, stirring frequently. Then boil for approximately 5 minutes without stirring. Remove from heat and stir in the baking soda. The mixture will foam up when the baking soda is added.
Place the popped popcorn on the baking sheets and remove any of the unpopped kernels. Pour this mixture over the popcorn and gently toss the popcorn to coat it with the mixture.
Bake the popcorn for 30 minutes, stirring the popcorn every 10 minutes.
Remove the popcorn from the oven and let the popcorn cool.