Add in the milk, flour and baking powder to a 12-oz or larger microwave-safe mug. Whisk together until smooth. Add in the Nutella and stir batter until fully blended, with no lumps.
Microwave on high for 60 seconds, then in 15 second increments, for 30 to 60 seconds more. Watch carefully for the cake to pull away from the sides of the mug, and the top to set. Once the top is dry and springs back when poked with a fork, your cake is ready.
Garnish with whatever you like, or top with a scoop of vanilla ice cream or whipped cream!