In saucepan over medium heat, combine milk, cocoa powder, sugar and cinnamon.
Heat, whisking constantly, until cocoa powder dissolves and mixture is hot.
If measured with food thermometer, temperature will be about 140 degrees.
Pour hot milk mixture into work bowl of food processor fitted with steel knife.
Process 30 seconds, until mixture is foamy.
Including foam, there will be about 2 cups of hot mixture.
Pour 1/4 c hot coffee into each of 4 cups.
Slowly pour 1/4 of the mixture over each cup of coffee.
The foam will remain on top of coffee.
Sprinkle with cocoa and serve immediately.
Additional sugar may be added, if desired.