Preheat oven to 350 degrees and grease a 9x5 inch pan, or line with parchment paper. In a bowl, whisk together the flour, baking soda and salt and set aside. In a large bowl, beat together the butter and brown sugar on medium-high speed until light and fluffy for about 2-3 minutes. Add the eggs, one at a time, beating well in between each addition. Add the yogurt, vanilla extract and mashed bananas and beat at medium speed until combined. Add the dry ingredients and mix on low speed until a batter forms. Once your batter is mixed, take two cups and move it to a separate bowl. Add the cocoa powder to that two cups and mix until combined. This is your chocolate batter. Take your regular batter and pour ¾ of it into the bottom of your pan. On top of this, put ¾ of your chocolate batter. Place the remaining ¼ of each batter on the top and swirl with a knife. Let the banana bread bake for 20-30 minutes and then place a piece of foil on it so the top doesn't get too brown/burn. Place back in the oven and let bake for another 30ish minutes. Total bake time is around 60-70 minutes depending on your oven. The banana bread is done when a toothpick inserted comes out clean and when the edges of the banana bread begin to peel away from the sides of the pan. Let cool and enjoy!