In a microwave-safe mug, combine the flour, sugar, cocoa powder, baking powder, and salt. Stir until well mixed.
Add in the milk, hazelnut spread, vegetable oil, and vanilla extract. Stir until a smooth batter forms, ensuring there are no lumps.
Sprinkle the chopped hazelnuts on top of the batter for added texture and flavor.
Microwave the mug on high for about 1 minute and 20 seconds. Check if the cake is set in the middle; if not, microwave in additional 10-second intervals until cooked through.
Allow the mug cake to cool for a minute before enjoying.
Notes
Serve warm directly in the mug or on a plate. Drizzle with extra hazelnut spread or whipped cream for an elevated dessert.