In a small bowl, whisk together the beaten egg, vanilla extract, oil, and milk until smooth.

Add the mashed banana and mix until fully incorporated.

In a microwave-safe mug, combine the flour, granulated sugar, brown sugar, and salt. Stir to remove any lumps.

Pour the wet ingredients into the mug with the dry ingredients. Gently fold the mixture until just combined.

Stir in the chopped walnuts (if using).

Microwave on high for 2-3 minutes, checking at 90 seconds. The cake is done when a toothpick inserted comes out clean.

Let the mug cool for 1-2 minutes before digging in. Top with whipped cream, a drizzle of caramel, or a sprinkle of cinnamon sugar for extra indulgence.