In a microwave-safe mug (around 12 oz capacity), combine the all-purpose flour, sugar, cocoa powder, baking powder, and salt. Whisk them together until well blended.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients. Stir until you have a smooth batter with no lumps.
Carefully spoon the chocolate hazelnut spread into the center of the batter in the mug. Do not mix it in; you want a gooey center.
If using, sprinkle the chopped hazelnuts on top of the batter.
Microwave the mug on high for 40-60 seconds. Start with 40 seconds and check the cake; continue microwaving in 10-second increments until the cake has risen and is set but still slightly moist.
Allow the mug cake to cool for a minute before enjoying it.
Notes
Serve warm with whipped cream or ice cream for an extra treat.