Pour 9 cups of Chex cereal into a very large mixing bowl, leaving enough room for gentle stirring. Set out one large or two medium zip-top bags for coating the cereal with powdered sugar later.
In a medium microwave-safe bowl, combine the chocolate chips, peanut butter, and butter. Microwave on HIGH for 30 seconds, then stir well. Continue microwaving in 20-second bursts, stirring after each, until the mixture is completely smooth and glossy (about 1–1 1/2 minutes total). Alternatively, melt gently in a saucepan over low heat, stirring constantly so it doesn’t scorch.
While the chocolate mixture is still warm, stir in the vanilla extract and fine sea salt until evenly combined. Taste a small dab (carefully, it’s hot) and adjust the salt if you’d like a slightly more salted chocolate flavor.
Pour the warm chocolate-peanut butter mixture over the cereal in the large bowl. Using a silicone spatula, gently fold and turn the cereal from the bottom up until every piece is evenly coated. Take your time and avoid crushing the cereal by stirring too aggressively.
Transfer the coated cereal to the large zip-top bag (or divide between two bags). Add 2 1/2 cups of powdered sugar to the bag, seal well, and shake vigorously, turning the bag in all directions so the sugar coats every piece. Check coverage; if you prefer a heavier, whiter coating, add up to 1/2 cup more powdered sugar and shake again.
Spread the coated cereal mixture onto a large baking sheet or parchment-lined counter in an even layer. Let cool for 15–20 minutes, or until the chocolate layer has firmed up and the powdered sugar coating adheres well. This helps the cereal stay crisp instead of turning slightly chewy.
Once the puppy chow is completely cool and dry to the touch, gently toss it with any desired mix-ins: M&M’s or candies, mini pretzels, mini marshmallows, nuts, and/or sprinkles. Fold gently in a bowl or mix with clean hands right on the baking sheet.
Transfer the finished puppy chow muddy buddies to an airtight container or serving bowl. Serve immediately, or store at room temperature until ready to enjoy.