Prepare the mug and all the ingredients.
Place the butter in the mug and microwave for 15 seconds to melt.Add the cocoa powder in the melted butter.
Crack the egg in the mug as well.
Stir thoroughly with a fork.Gradually combine all of the remaining ingredients (except the toppings) into the mug while stirring.
Fold everything together until smooth.Remember to fill only 2/3 of the cup to prevent spilling the batter when cooking.Put the mug in the microwave for another 1.5 - 2 minutes depending on your mug size.
Make sure to check to prevent the cake from becoming too dry or burnt.If desired, top with walnuts and sour cream.
Serve and enjoy!