Combine espresso powder, water and sugar (if using) in a coffee mug. Set aside.
Meanwhile, heat milk in a microwaveable cup for 30 seconds ( for milk at room temperature) or for 50 seconds (for refrigerated milk). When milk is really warm, pop on the lid, and shake vigorously for 60- 80 seconds. This technique is used to create the foam.
Remove the lid. Place milk back in the microwave. Heat for another 30 seconds. The foam will set.
Use a spoon to move the foam backwards, and pour the heated milk into espresso mixture. Gently place the white foam on top. garnish with cocoa powder, if you like.