Dark chocolate banana swirl muffins

There's 3 left-over bananas after my banana chiffon cake, and I had to put them in the fridge as they had over-ripened. I was thinking of something chocolate with the banana, and chance upon a yummy-looking picture here. And here's my version. 

I'm copying the recipe here for future reference.

Ingredients (makes about 12 muffins)

• 1 1/2 cups all-purpose flour
• 1 teaspoon baking soda
• 1 teaspoon baking powder
• 1/2 teaspoon salt
• 3 bananas, mashed
• 1/4 cup white sugar
• 1/2 cup brown sugar
• 1 egg, lightly beaten
• 1/3 cup vegetable oil
• 1/2 tsp vanilla
• 1/4 cup dutch process cocoa powder
• 3 tablespoons boiling water
• chocolate chips for topping

Instructions

1. Preheat oven to 375 degrees F (I use 185 deg C) 
2. Lightly grease 12 muffin cups, or line with muffin papers.
3. In a bowl, mix together flour, baking powder and baking soda
4. In a large bowl, mix together bananas, sugars, egg and oil, and vanilla. Gradually add the flour mixture into the banana mixture just until moistened.
5. Spoon 1/3 of the muffin batter into another bowl. Stir the cocoa powder and boiling water together until smooth, and then stir the cocoa mixture into the batter that you set aside.
6. Spoon a dollop of regular into each muffin well- just enough to cover the bottom. Follow with a dollop of chocolate batter, another small spoon of regular and then a small little spoon of chocolate, filling to a maximum of 3/4 of the way.
7. Use a toothpick or skewer to swirl all the batter together, and sprinkle a few chocolate chips on top.
8. Bake for 20-25 minutes or until muffins are golden and a toothpick inserted into the center comes out clean.