Preheat your oven to 200°F and line a large rimmed baking sheet (18" x 13") with parchment paper. Pour the popped corn into a large bowl (at least 6 quarts); remove any unpopped kernels. Set aside.
In a large saucepan over medium-high heat cook the molasses, brown sugar, butter, and corn syrup, stirring occasionally, until the sugar dissolves. Stirring occasionally, boil the syrup for 5 minutes. The syrup will darken slightly.
Remove the pan from the heat and add the baking soda. Stir well as the mixture turns foamy.
Immediately (and carefully) pour the hot syrup over the popped corn and stir until the corn is well coated.
Spread the hot caramel corn into the parchment-lined pan. Bake at 200°F for one hour. Stir the corn every 15 minutes during this time.
Remove from the oven. This caramel corn is a warm, sticky, sweet and crunchy treat straight from the oven. It can also be cooled and stored airtight for several days. This recipe doubles and triples very well.