Measure out the cereal. Pour in a large bowl, set aside.

Melt butter in a heavy saucepan over medium heat. Add sugar.

Stir constantly for 6-8 minutes until the color is light brown (like a paper bag). If using a candy thermometer the temperature should reach about 298°F. The mixture may look curdled or separated- that's okay. You are looking for a light brown caramel color.

Immediately remove from heat and slowly add vanilla- being careful to avoid any splatters to your skin. Stir for a few more seconds, and pour over the cereal.

Stir the toffee to coat the cereal as best you can then spread the mixture onto your prepared baking sheet to cool. Add M&Ms if desired. Store in airtight container for up to 1 week.