Brew 2 shots of espresso using an espresso machine or stovetop espresso maker.
In your serving mug, combine the dark brown sugar and chocolate sauce, stirring together to form a paste.
Pour the hot espresso over the chocolate-brown sugar mixture and stir vigorously until completely dissolved.
Heat the milk and heavy cream together until steaming, either in the microwave for 45-60 seconds or in a small saucepan over medium heat.
Froth the heated milk mixture using a milk frother or whisk until foamy, then pour over the espresso mixture.
Top with whipped cream if desired and serve immediately.