In a large microwave-safe bowl, combine chocolate chips and butter. Microwave in 30-second intervals, stirring each time, until fully melted and smooth.

Stir in vanilla extract.

Gently fold in the Chex cereal until evenly coated with the chocolate mixture.

In a separate large zip-top bag or bowl, mix powdered sugar with Dutch-processed cocoa powder.

Add the coated cereal to the bag and shake until all pieces are fully covered.

Spread mixture on a baking sheet to cool and set completely.

Once set, mix in mini chocolate chips and optional brownie bites or crumbs.

Store in an airtight container at room temperature or refrigerate.