Steps
1. In a microwave-safe mug, mash the ripe banana with a fork until smooth.
2. Add the sugar, melted butter, vanilla extract, and egg yolk, and mix until well combined.
3. Stir in the flour, baking powder, and salt until just incorporated.
4. If desired, fold in chopped walnuts or chocolate chips.
5. Microwave on high for 1-1.5 minutes, or until the cake is set and a toothpick comes out clean.
6. Let it cool slightly before enjoying your banana bread mug cake!