In a microwave-safe mug, combine flour, sugar, cocoa powder, and baking powder.
Mix dry ingredients well to avoid clumps.
Add milk and vegetable oil to the mug.
Stir until the mixture is smooth and all ingredients are incorporated.
Add Nutella and mix thoroughly until well combined.
Microwave on high for about 1 minute.
Check if the cake is cooked through; if not, microwave for an additional 10 seconds.
Let it cool for a minute before serving.
Top with whipped cream or ice cream if desired.