Break a room temperature egg in 12 oz microwave safe mug and whisk it with sugar, vanilla and salt until well combined. Next add in a mashed banana, melted butter and give it a quick mix too.
Sift flour, baking powder and cinnamon over wet ingredients right in the mug and fold to combine. Add dry ingredients in two additions for easy mixing.
Next add in remaining flour, a tablespoon of milk, walnuts and chocolate chips. Stir them all in. Top with a few chocolate chips and a sprinkle of cinnamon. Microwave the batter for 3 minutes. Check after 2 minutes for doneness in between. If there is no wet batter in sight, skip cooking it for one more minute.
The cake will rise above the rim of your mug as it cooks, but it won't spill. Just make sure to use a large sized mug to avoid spilling. Once cooked, carefully remove it from microwave, make use of gloves as the cup will be very hot.
Allow it to cool for 1-2 minutes in the mug itself. Top with banana slices, a dallop of butter, some honey/maple syrup and more chocolate chips. Dig in!