My dad always made what we called candy corn when we were kids. It was nothing more than hard candy poured over popped corn. I'll share his recipe another day. This recipe is from Grandma's collection. It is different from Dad's in a couple ways.
1/2 cup syrup
1 cup white sugar
1 t vinegar
1/4 t salt
1 t butter
1 t soda
Mix sugar, vinegar, syrup, salt, butter and bring to boil. Stir constantly. Boil gently to hard cook in water (290o) Add soda mix well. Pour over popped corn and stir to mix well.
Notes. The syrup would be a Karo or corn syrup. Since this is called caramel corn and since the sugar is not 'burnt' I am guessing she would have used dark corn syrup.
I am a bit puzzled by the vinegar/soda part of this recipe. That is a standard technique for making the candy foam and is used in things like peanut brittle. It may be used here to expand the sugar so it coats the corn better. It may be worth some experimentation.
One more note... It is a good idea to sift or scoop out as many unpopped kernels as you can before you add the syrup.
As an aside... This recipe is written on a small piece of paper that is torn off a perforated pad. I remember playing with paper like this and being fascinated by the perforations.
And some comments from my sister when I asked her about the vinegar and baking soda.
Peanut brittle has the baking soda but not the vinegar. When it is hot, the baking soda bubbles up and I think it makes the candy lighter. That is probably the case with the vinegar and soda in the caramel corn recipe. You don't want to break a tooth and since the temperature control was iffy on a wood stove, that may have been a solution. Dad did not do it that way. He didn't even take it up to caramel temp., probably because it could get too hard - that's why we had pink or whatever color "caramel" corn!There could be another reason: for nutrition, it might be the vinegar was a source of vit C or something necessary to prevent disease and they added it to everything - especially the Germans!