Prepare the Banana
Using a fork, mash the banana in a small bowl until it reaches a smooth, pulp-like consistency.
Incorporate the Wet Ingredients
Add the melted butter, egg, and vanilla extract to the mashed banana. Stir until well combined, ensuring a smooth mixture.
Combine the Dry Ingredients
Gradually mix in the sugar, flour, baking powder, baking soda, and a pinch of salt. Fold in the dark chocolate chunks. Stir gently to incorporate all ingredients without overmixing—ensure no dry patches remain in the batter.
Microwave the Mug Cake
Transfer the batter into a microwave-safe mug with a capacity of at least 12 ounces. Heat in the microwave for 1 minute and 30 seconds, then continue in 10-second intervals until the cake is fully set. The top should be slightly sticky but not wet, and the cake will expand significantly as it cooks.