In a microwave-safe mug, combine the all-purpose flour, granulated sugar, cocoa powder, baking powder, and salt. Mix well to ensure there are no lumps.
Add the milk, vegetable oil, and vanilla extract into the dry ingredients. Stir until smooth and fully combined.
Drop the chocolate hazelnut spread into the center of the mixture. Gently swirl it in with a fork but do not completely mix it in—this will create a melty chocolate center when baked.
Microwave the mug on high for 1 minute and 10 seconds. (Cooking times may vary based on your microwave; keep an eye on it to prevent overflowing.)
Once done, carefully remove the mug from the microwave (it will be hot!) and let it cool for a minute.
Top with chopped hazelnuts for added crunch. You can also add a dollop of extra chocolate hazelnut spread on top for extra indulgence if desired!
Notes
Microwave times may vary; watch closely to prevent overflow.