Line a baking sheet with aluminum foil and lightly brush or spray with oil.  I like using the non-stick aluminum foil.  If you are going to bake the popcorn at the end you will need to preheat the oven to 350 degrees.

Make popcorn either on the stove or in the microwave.  If using microwave popcorn make sure buy plain, not buttered or flavored.  If using the stove top method then heat the oil over medium-high heat in a deep pan with 3 kernels in it.  Once the kernels pop add the remaining 1/2 cup of kernels.  Keep shaking the pan until you hear 3 seconds of NO popping.  Immediately put popped corn on prepared baking sheet.

Cube the butter so that it melts faster.  Melt butter over medium heat, then add salt, brown sugar and corn syrup.  Stir and let lightly simmer for about 3-5 minutes.  You are looking for a nice brown color and a sweet smell.  Remove from heat and add baking soda.

Combine peanuts with popcorn, then pour caramel over the tray.  Spary a spoon or spatula with oil and use it to mix the popcorn, peanuts and caramel.  Keep stirring until evenly coated.

You can eat the Cracker Jack once it's cooled or you can bake for 15-60 minutes to harden the corn which will make it last a couple of days longer.  If you bake the Cracker Jack you will want to stir every 10-15 minutes.  If you are eating right away or by the next day the additional cooking isn't necessary.

Once the Cracker Jack is cool enough, break into bite size pieces or cut into bars.  Enjoy!