In a large mixing bowl, mix butter and sugar until blended and fluffy, approx. 3 minutes on low-medium mixing speed.
Mix in one egg at a time. Add mashed bananas, vanilla extract, and yogurt. Mix until blended.
Add all dry ingredients into the banana mixture (except for chocolate and cocoa powder), mix until fully combined.
In a separate microwaveable bowl, melt the semi sweet chocolate and mix in cocoa powder. Add 1 cup of the banana mixture into this chocolate mixture and mix until the chocolate batter is smooth and silky.
In a greased loaf pan, fill the pan alternating between the two batters. A loaf pan can have two side by side rows of 4-5 Tbsp scoops per row for the first layer, and the same for the second layer. Does not have to be perfect, alternating is key.
Use a knife and run through the batter in the loaf pan, crisscrossing to semi mix the two mixtures to form the marbled look. Around 4 times and do not over mix.
Bake at 350 F for 1 hour or until tester in the centre of the loaf comes out clean.