Add butter to a cereal size, microwave-safe bowl. Heat in microwave just until butter melts (around 30 seconds).
Add sugar and vanilla extract. Stir until well combined.
Use a fork to mash the banana into the mixture until smooth.
In a separate bowl, mix together flour, baking powder, cinnamon and salt. Then, sprinkle dry ingredients evenly over wet ingredients.*
Add milk & mix until everything is well combined and large lumps are gone.
Clean batter from the edges of bowl, then microwave on full power for 2 minutes (up to 2 minutes and 30 seconds), or just until toothpick inserted in middle comes out clean. Cooking time may differ based on strength of your microwave. Do not overcook.**
Allow cake to cool a little, then add toppings and enjoy!