Preheat your oven to 350°F (if using a dark-coated tin, lower to 325°F). Line a muffin tin with paper liners. (This batch makes about 18 muffins so bake in two rounds if needed.)

In a large mixing bowl, combine mashed bananas, granulated sugar, brown sugar, oil, sour cream, eggs, and vanilla. Whisk until the mixture is smooth and well blended.

Add the flour, baking powder, salt, and baking soda. Stir gently just until there are no dry pockets remaining. (It’s perfectly fine if your batter remains a bit lumpy.)

For the chocolate component, whisk together cocoa powder and boiling water in a medium bowl until the mixture is velvety smooth. Then, carefully fold in about 1 1/4 cups of the banana batter into the cocoa mixture. Mix gently until fully incorporated but do not over mix.

Layer the batters into each muffin cup as follows: spoon approximately 1 1/2 tablespoons of regular banana batter, followed by a scant 1 tablespoon of chocolate batter, then another 1 1/2 tablespoons of the plain batter, and finally finish with a teaspoon more of chocolate batter. (Remember, you’re layering, not stirring!)

If desired, lightly dust the tops with coarse sugar before baking.

Bake for 17-19 minutes, or until the muffin tops spring back when lightly pressed. Remove from the tin and let them cool completely on a wire rack.