Preheat oven to 350F and lightly grease a 12-cup bundt pan (12-inch diameter).
In a large bowl, cream together butter and sugar until light. Beat in the eggs one by one, waiting until each has been incorporated to add the next. Beat in vanilla extract and mashed bananas.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Add half of the flour mixture to the butter mix and stir to combine. Add in the yogurt, followed by the remaining flour mixture.
Remove 1 1/2 cups of batter into a small bowl and stir in cocoa powder.
Pour 2/3 of the plain batter into the prepared bundt pan, top with cocoa batter and spoon the rest of the plain batter on top. Run a knife through the batter gently to swirl.
Bake for 45-55 minutes, or until a toothpick inserted into the center of the cake comes out clean and the top springs back when lightly pressed.
Turn cake out onto a wire rack to cool completely before slicing.
Serve plain, or dusted with powdered sugar.
To make muffins, follow steps above filling 2 lined muffin pans. Bake at 350 for 20-25 minutes, or until they test done.