Melt the coconut oil in a large pot. When hot, test it by adding three popcorn kernels to the pot. When the kernels pop, add the rest of the kernels to the pot. Swirl the pot to coat all the kernels.
Cover with a lid slightly ajar to let out steam, shaking the pot every so often as the kernels pop. When the popping slows (1 to 2 seconds between pops), remove from heat and shake another 30 seconds or so until all the popcorn is popped.
Pour half the melted butter + parmesan + rosemary over the popcorn. Toss to coat. Add the rest of the butter, parmesan + rosemary, toss to coat again, then add sea salt as needed.