In a large skillet (preferably non stick), melt the butter over medium heat. Once melted, add the soy sauce and use a heat proof spatula or wooden spoon to give it a quick mix. Add the cereal and pretzels to the skillet, and toss to coat them well with the butter mixture. Sprinkle the nutritional yeast mixture over the cereal, tossing well. Reduce heat to low and cook for several minutes to toast the cereal. Remove to a large bowl, add sprinkle with pepper if using. Let mixture cool and serve. It will continue to crisp up as it cools.