Place milk in a 2-cup microwave-safe measuring cup. Microwave, uncovered, on high until milk is hot and small bubbles form around edge of cup, 1-2 minutes.
Divide condensed milk among four glass mugs; carefully pour coffee over milk.
Place a metal whisk in warm milk; whisk vigorously by holding whisk handle loosely between palms and quickly rubbing hands back and forth to create foam. Remove foam to another 2-cup measuring cup as it forms. Continue whisking milk until foam measures 2 cups. Spoon foam over coffee. Discard any remaining milk. Sprinkle foam with cinnamon; serve with a spoon.