Line a baking sheet with parchment and spread the popcorn out in an even layer. Set aside.
Melt the butter in a medium saucepan over medium-high heat. Stir in the sugar and corn syrup.
Bring the mixture to a boil stirring constantly. Turn the heat to low and allow to simmer, untouched for about 4-5 minutes.
Stir the sauce and then continue cooking for another 4-5 minutes stirring occasionally until the mixture turns a deep golden brown.
Turn off the heat. Whisk in the salt, baking soda, cream of tartar and sauce being careful because the sauce will bubble up.
Immediately pour the caramel mixture over the popcorn and toss to coat. (This is easier to do if your baking sheet and popcorn are warm so you can pop the whole sheet into the oven for a couple minutes before this step.)
Transfer the baking sheet to the oven and bake for 30-40 minutes, stirring ever 15-20 minutes.
Remove the baking sheet from the oven and allow the popcorn to cool.
Break apart the caramel popcorn and transfer to an airtight container. Store at room temperature.