In a small bowl, mix together all the ingredients for the churro sugar coating (powdered sugar, granulated sugar, and cinnamon), then set aside.
Cinnamon Coated Popcorn
Prepare popcorn by microwave for 3-4 minutes (or until popping slows). Pour popcorn in a large mixing bowl and set aside, giving it time to cool a little while you prepare the next steps.
In a medium saucepan, melt butter over medium heat. Add granulated sugar, cinnamon, light corn syrup, and vanilla. Allow ingredients to a simmer, whisking constantly, until mixture is smooth and no longer grainy, about 2-4 minutes.
Begin to slowly pour cinnamon coating over popcorn a little at a time. Use a spatula to gently stir and toss the popcorn to coat.
Sprinkle the top of the popcorn with 1/3 of the churro sugar coating, then toss to coat. Continue adding more churro sugar coating until popcorn is completely covered and appears light gold in color.