Whisk in the milk and vegetable oil until all ingredients are combined and batter has no clumps.
Pour batter into a microwave-safe mug. I used a mug which is approx 1 1/4 cups. You want enough head space for the cake to rise without pouring over.
Add 1 tbsp. of hazelnut chocolate spread (Nutella) / Peanut Butter in the middle of the batter. Just drop it in the middle, no need to push it down and sink it in the batter.
Place a paper towel into the microwave and set the mug on top (this is to catch any batter if your mug cake overflows).
Microwave mug cake for 70 seconds on high (PLEASE READ NOTES CAREFULLY BELOW)
Carefully remove from microwave and enjoy!
Notes
Please note that ALL microwaves are different therefore can yield different results and can overcook this mug cake if you aren't careful.I cooked mine on 100% power for 60 seconds and it had a gooey fudgy texture. If you prefer a more set texture, MW for 70 seconds. Knowing this information, it may help you better judge how long to cook yours for and at what power.If you are dividing the above qty between two bowls (I used bowls which are approx 1 cup each), MW on 100% power for 50 seconds first, test with a toothpick and cook for 10-20 seconds more, if required.I think it's better to rest the cake for 5 mins before serving it.