Butter the sides of a heavy 2-quart saucepan. Combine sugar, sorghum molasses, and cream in prepared saucepan. Cook and stir over medium-high heat to boiling.
Clip candy thermometer to side of pan. Cook over medium heat, stirring frequently, until the thermometer registers 240 degree F (soft-ball stage). Mixture should boil evenly over entire surface. Remove the pan from heat; remove thermometer. Stir in baking soda.
Slowly pour mixture over the popped popcorn. Stir until mixed.
Working quickly, shape with buttered hands into 2-1/2- to 3-inch balls. Wrap with plastic wrap or waxed paper. Makes 13 to 15 balls.