Preheat oven to 250ºF. Mist 2 large, rimmed baking sheets with cooking spray.
In a large, heavy-bottomed pot, warm oil over medium heat. Add 3 popcorn kernels and cover until they pop. Add remaining kernels, cover and shake pan continuously over burner until popcorn is completely popped, 2 to 4 minutes. Transfer popcorn to a large bowl, discarding any unpopped kernels. Toss pepitas into bowl with popcorn.
In a medium pot over medium heat, combine butter, sugar and corn syrup. Cook, stirring occasionally, until sugar has dissolved. Increase heat to medium-high and cook, stirring, until mixture turns golden brown, about 5 minutes. Remove from heat and quickly stir in salt and cayenne with a wooden spoon. Pour over popcorn mixture and toss to coat evenly.
Divide popcorn mixture between baking sheets. Bake for 30 to 35 minutes, stirring occasionally. Let cool on baking sheets on a wire rack until cool enough to break into small chunks.