Add the Chex cereal to a large mixing bowl and set aside.
In a microwave-safe bowl, combine the white chocolate chips, cookie butter, and unsalted butter. Microwave in 20-second intervals, stirring between each, until smooth and melted.
Stir in the vanilla extract.
Pour the melted cookie butter mixture over the Chex cereal and gently fold until evenly coated.
Transfer the coated cereal to a large zip-top bag or lidded container. Add 1 cup of powdered sugar and shake gently until coated.
Add the remaining ½ cup powdered sugar as needed and shake again until the cereal is no longer sticky.
Spread the puppy chow onto a parchment-lined baking sheet and let set for 15 minutes.
Gently toss with crushed Biscoff cookies, if using, and serve.