In a medium saucepan, bring oil to a simmer over medium-low heat, stirring often.

Add popcorn kernels in a single layer and cover saucepan with a lid. When the popcorn begins to pop (after about 2-3 minutes), shake the pot over the heat for about 3 minutes, until popping slows. Remove from heat immediately and let cool.

In another medium saucepan, melt butter on medium-high heat. Add sugar, corn syrup, cream of tartar and salt. Stirring constantly, bring mixture to a boil for about 5 minutes. Working quickly, remove saucepan from heat and stir in the baking soda. Mixture will become thick and bubbly. Pour the caramel mixture over the popcorn and toss gently until all the kernels are coated.

Place baking sheets in the oven and bake for 30 minutes. Remove pans one at a time and stir each one again before heating for another 30 minutes.

Remove from oven and sprinkle with sea salt. Let the popcorn cool on the pan. If desired, break apart any large clusters. Store in an airtight container for up to 2 weeks.