Place the Chex cereal in a large mixing bowl and set aside.
In a microwave-safe bowl, combine the white chocolate, butter, lemon zest, and lemon juice.
Microwave for 1 minute, then stir. Continue microwaving in 30-second intervals, stirring after each, until the mixture is smooth and fully melted.
Pour the warm lemon mixture over the cereal and gently fold until every piece is evenly coated.
Add the powdered sugar and gently toss until the cereal is completely coated. A large zip-top bag also works well for this step.
Spread the puppy chow onto parchment paper or a parchment-lined baking sheet in an even layer.
Let cool completely for at least 1 hour before storing in an airtight container. Letting it cool completely prevents the snack mix from getting soggy or stale prematurely.
Serve immediately or store in an airtight container for later snacking.