Pour 9 cups of Chex cereal into a very large mixing bowl so you have room to toss without breaking the cereal. Add 3 cups of powdered sugar to a large 2-gallon zip-top bag (or divide between two 1-gallon bags). Set both aside.

In a microwave-safe bowl, combine the semisweet chocolate chips, creamy peanut butter, and butter. Microwave on 50% power for 30–45 seconds at a time, stirring after each burst, until the mixture is smooth, glossy, and fully melted, about 1 1/2–2 minutes total. Alternatively, melt together in a double boiler over low heat, stirring constantly.

Let the melted chocolate mixture sit for a few seconds so it is very warm but not scalding, then stir in the vanilla extract until fully combined.

Pour the warm chocolate-peanut-butter mixture over the Chex cereal in the large bowl. Using a wide rubber spatula, gently fold from the bottom up until every piece of cereal looks evenly coated and shiny, taking care not to crush the cereal.

Transfer the chocolate-coated cereal to the bag or bags with powdered sugar. Seal the bag tightly, pressing out excess air, then shake, flip, and roll the bag for 30–60 seconds until the cereal is evenly coated and looks pale. If you still see patches of dark chocolate, add an additional 1/4–1/2 cup powdered sugar and shake again.

Line one or two large baking sheets with parchment or wax paper. Pour the coated puppy chow onto the sheets and spread into an even layer. Let cool at room temperature for 20–30 minutes until the chocolate is set, or chill in the refrigerator for 10–15 minutes if your kitchen is warm.

Once completely cool and set, gently stir in any desired add-ins such as M&M’s, mini pretzels, mini chocolate chips, or nuts. Transfer to a serving bowl or airtight container and serve.