Melt the butter in a medium saucepan. Do not brown. Add brown sugar and Karo syrup and stir to combine. Bring to a boil and cook to soft ball stage. Test in a cup of very cold water. Add the Eagle Brand milk and stir just until blended. Add the baking soda and stir. Mixture will froth.
Immediately pour over 4 to 5 quarts of popped corn and stir until it is covered evenly.
Pour onto a piece of waxed paper to dry slightly or make immediately into popcorn balls.