Preheat the oven to 250 degrees F. Spray a baking sheet with nonstick spray.
To make the spice mix, whisk together the buttermilk powder, garlic powder, onion powder, dried chives, dried dill, dried parsley, dried oregano, dried basil, paprika, salt and pepper in a small bowl. Set aside.
In a separate bowl, mix the butter, sugar and mustard until combined. Set aside.
Add the cereal squares, pretzel sticks, nuts, pita chips and cheese squares to a large bowl. Add all but 2 tablespoons of the melted butter mixture and toss to coat evenly. Sprinkle with all but 1 tablespoon of the spice mix and toss to combine.
Spread the coated cereal mix onto the prepared baking sheet. Bake, stirring every 10 to 15 minutes, until toasty and crispy, about 50 minutes.
When the mixture comes out of the oven, pour the popcorn into a large bowl and toss with the remaining butter mixture and seasoning mixture. Add the toasted cereal mix and mix thoroughly. Serve immediately or store in an airtight container for up to 5 days.