Place the 9 cups of Chex cereal in a very large mixing bowl, choosing one that leaves enough room for gentle folding. Set the bowl near your work area so the warm coating can be added immediately after melting.

Add the semisweet chocolate chips, creamy peanut butter, and unsalted butter to a microwave-safe bowl. Microwave in 30-second intervals, stirring thoroughly after each interval, until the mixture is smooth; the melting process generally takes 1 to 2 minutes total.

Stir the vanilla extract into the smooth chocolate mixture. Pour it over the cereal and fold gently with a spatula until the pieces are evenly coated, taking care not to crush the cereal.

Transfer the coated cereal to a large resealable plastic bag or a roomy covered container. Add the powdered sugar, seal securely, and shake until every piece has a generous, even coating.

Spread the finished mixture over waxed paper or parchment in an even layer. Let it set for 15 to 30 minutes before serving, then store any leftovers in an airtight container.