Pop the kernels until you have 24 cups of popped corn. Separate the un-popped kernels and discard.
Fill a small bowl with some ice water and set next to the stove top.
In a large sauce pan combine white sugar, brown sugar, corn syrup, milk, butter, and the salt. Place over high heat stirring constantly.
Once the caramel comes to a boil, cook for another minute or two and then remove from heat.
Spoon a teaspoon or so of the caramel into the bowl with the ice water. Try to form the caramel into a ball. If it holds together it is done cooking. If the caramel falls apart return the sauce pan to the heat and continue cooking, stirring continuously for another minute or two before testing again.
Once the caramel reaches soft ball stage remove from heat and stir in 1 tsp. of vanilla
Carefully pour the caramel over the popped corn.