In another bowl, mix together all ingredients from banana to yogurt till smooth. Add into the mug and fold till all moist.
Add the crushed walnuts and just fold.
Microwave for three minutes on full powder and allow it to rest for two minutes. Dig in.
Notes
My microwave is 900W so the cake was done in three minutes. If it looks uncooked after the resting period, microwave for another 30 seconds, rest and enjoy.