Preheat oven to 250 degrees.
Add all of the dry mix ingredients into a large bowl and set aside. In a saucepan, melt the butter over medium-low heat, whisk in the rest of the sauce ingredients until well blended, remove from heat.
Pour the sauce over the cereal mix. Toss gently with a wooden spoon until all coated. Divide onto two large baking sheet pans. Spread into a somewhat flat layer so that it bakes evenly. Bake for 20 minutes, then stir and rotate pans. Bake for another 20 minutes, and stir and rotate. Bake for a final 5-10 minutes or until crisp.
Spread onto foil or paper towels to cool completely. Store in an air tight container.
Combine brown sugar and mustard powder and sprinkle over hot chex, just as it comes out of the oven. I'm indifferent on this step, but it gives you more of that outer layer of seasoning.
Add in any extra crunchies that you like, sometimes I use goldfish crackers or Cheerios, but they could get soggy in this recipe since they are so small. The bran sticks and sesame sticks I find my the bulk nuts and candy section of my grocery store.
The optional seasoning at the end doesn't necessarily add any flavor, it is more for that exterior powder seasoning feel that you get from packaged snacks if you're into that.