In a microwave-safe mug (around 12 ounces), combine the flour, cocoa powder, baking powder, sugar, and salt. Mix well with a small whisk or fork until thoroughly combined.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until you have a smooth batter with no lumps.
Gently fold in the chocolate hazelnut spread, making sure it’s well incorporated but with some swirls remaining for a marbled effect. If desired, add in the chopped hazelnuts for an extra crunch.
Microwave the mug on high for about 40-60 seconds. Start with 40 seconds and check the doneness; if it's still gooey in the middle, continue microwaving in 10-second intervals until fully cooked.
Let the mug cake rest for a minute before enjoying, as it will be extremely hot.
Notes
Serve warm with whipped cream or ice cream for an extra treat.