Place butter and eggs on the counter to bring to room temperature.
Preheat oven to 350 degrees and line your muffin pan with papers.
In a medium bowl whisk together dry ingredients.
In the bowl of your electric mixer fitted with the paddle attachment beat together butter and sugar until light and fluffy. Add eggs one at a time scrapping down the sides of the bowl after each addition.
Mix in mashed bananas, yogurt and vanilla. Add dry ingredients and stir on low just until combined.
In a glass 2 cup measuring cup melt chocolate chips in the microwave. 1 min, stir, 30 seconds and stir in the cocoa powder.
Remove one cup of the banana batter and stir into the chocolate mixture.
Use a spatula to ensure the banana batter is completely mixed. Drop spoonfuls of the chocolate batter into the banana and swirl slightly. Do not over mix or batters will become blended. Using a muffin scoop fill your pan. You may be able to make 14 muffins.
Bake until a toothpick comes out clean approximately 25 minutes.