Position an oven rack in the bottom of the oven, and preheat it to 200 degrees F.
Line a baking sheet with parchment and generously coat it with cooking spray. Spread the tomato paste into a thin layer on the parchment (an offset metal spatula works best). Bake until the paste deepens to brick red and is dull in color and not shiny in any spots, about 2 hours. (It will look and feel like fruit leather.) Let it cool completely. Tear the paste into small pieces and finely grind them in batches in a spice grinder.
Place all the ground tomato paste back into the spice grinder. Add the Parmesan, oregano, garlic salt and pepper flakes, and grind to a fine powder. Store in an airtight container for up to 5 days. The spice blend makes enough for about 6 batches of popcorn.
To make 1 batch of pepperoni pizza popcorn: Heat 1/2 tablespoon olive or vegetable oil in a large pot over medium heat. Add 1/2 cup kernels; when one pops, cover the pot, and cook, shaking the pot constantly, until the popping subsides to 1 pop every 20 seconds, about 5 minutes. Add the pepperoni, sundried tomatoes, some melted butter and 2 heaping tablespoons of the pizza spice and toss to coat.