In a microwave-safe mug (approximately 12 oz capacity), whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients, then stir until you have a smooth batter. Make sure no lumps remain.
Dollop the chocolate hazelnut spread on top of the batter. Use a toothpick or small spoon to swirl it into the mixture without completely mixing it in. This will create a melty chocolate center.
Microwave the mug on high for about 40-60 seconds. The cooking time may vary based on your microwave's wattage, so keep an eye on it. The cake should rise and be set but slightly moist in the center.
Let the mug cake cool for a minute before enjoying. If you like, top with chopped hazelnuts for added crunch and flavor.