In a small bowl whisk together the heavy cream and espresso powder until the espresso powder is dissolved. Set aside.
In a microwave safe bowl, melt the butter. Add the sugar, egg yolk, vanilla and the cream/espresso mixture and whisk until the mixture is smooth.
Whisk together the flour, baking powder and salt and add it to the mixture. Mix together using a small rubber spatula. Fold in the chocolate chips.
Divide the batter evenly beween the 4 cups. Microwave all 4 cups, at the same time, for 40 seconds.
Let them sit for 1-2 minutes before garnishing with slightly sweetened whipped cream.
Enjoy!