Place rice chex into a large bowl. Set aside. In a small bowl, stir together granulated sugar and cinnamon.
In a double boiler, melt butter and chocolate together until completely smooth. Pour over cereal and stir to coat as evenly as possible. Do your best to work quickly so everything gets coated as evenly as possible.
Sprinkle cinnamon sugar overtop the coated chex and toss to coat. Spread onto a parchment paper lined baking sheet to harden. Refrigerate 5 minutes. Store in airtight container for up to 5 days at room temperature or 10 days in the refrigerator.
Alternately, you can make this like a classic chex mix and use a large plastic bag to coat cereal in white chocolate and then toss with the cinnamon sugar. (I just prefer a bowl.)