Make your espresso or strongly brewed coffee: As mentioned in the notes above, any strongly brewed espresso from a home espresso maker or a capsule machine like a Nespresso will do. Or use strongly brewed coffee from an Aeropress, not diluted with any water.
Froth the milk: Pour milk into the jar. Fill no more than halfway. Froth the milk until the milk is frothy and has roughly doubled in volume.
Microwave the milk: Take the lid off the jar and microwave uncovered for 30 seconds. The foam will rise to the top of the milk and the heat from the microwave will help stabilize it.
Pour warm milk into the espresso: Pour the espresso or coffee into a wide, shallow coffee cup. Use a large spoon to hold back the milk foam, and pour as much warm milk as you would like into the espresso.
Add foam: Spoon as much milk foam as you would like onto your latte (or perhaps it's a cappuccino at this point!). Garnish, if desired, with a sprinkle of cocoa powder or nutmeg on top of the foam. Sip immediately!