Preheat oven to 350°F and line a loaf tin with parchment paper.
In a mixing bowl, mash the bananas until only a few small chunks remain. Add in the brown sugar, granulated sugar, eggs, melted butter, greek yogurt, and vanilla extract and whisk together until well combined.
In a separate bowl, stir together the flour, baking soda, and salt. Add this flour mixture to the wet mixture and fold together with a spatula until just combined. Don't overmix.
Separate out about 300g of batter into a separate small bowl and set aside.
In a small saucepan on low-medium heat or microwave, melt the dark chocolate. Pour the melted chocolate into the small bowl of batter that you separated out and quickly mix together.
Pour half of the banana bread batter into the loaf tin and spoon blobs of chocolate batter on top. Repeat until you use up both batters. Use a chopstick or knife to swirl the batters together.
Bake for 1 hour or until a toothpick inserted comes out almost clean. Let the banana bread cool for at least 10-15 minutes before removing from the baking tin and finish cooling on a wire rack.