Combine the butter, fish sauce and sriracha in a microwaveable container and microwave on high until melted, about 30 seconds.
In large bowl, toss the cereals with the powdered Cheddar. Add the melted butter mixture and mix to coat. Add the nuts and cilantro and mix again.
Microwave the cereal mixture on high for 6 minutes, stopping to stir every 2 minutes.
Add the sour gummy candies to the cereal mixture and stir. Spread the hot mixture on a cookie sheet in an even layer. Top with an even layer of pretzels. Let rest until cooled.