Preheat the oven to 350°F (175°C). Grease a 9x5 inch loaf pan and line it with a parchment sling for easy lifting.
In a small bowl, mix the swirl ingredients: brown sugar, cinnamon, and flour. Set aside.
In a large bowl, whisk the melted butter with the brown sugar and granulated sugar until it looks glossy. Add the eggs and vanilla, whisk again. Stir in the mashed bananas and the yogurt until just combined. Breathe. It doesn’t need to be perfect.
In a separate bowl, whisk the flour, baking soda, salt, and cinnamon. Add these dry ingredients to the wet mixture and fold gently with a spatula until you no longer see dry pockets. If using nuts or chocolate chips, fold them in now. The batter will be thick and cozy-looking.
Layer the batter: add about half to the pan, smooth it out. Sprinkle on half of the cinnamon swirl. Use a butter knife to lightly ripple it through - a few figure eights do the trick.
Add the rest of the batter, then the remaining swirl. Ripple again, gently. You’re aiming for a ribbon, not a whirlpool.
Bake for 55 to 65 minutes, until a toothpick inserted near the center comes out mostly clean with a few moist crumbs, or the center reads about 200°F (93°C). If the top is browning fast, tent loosely with foil for the last 10 to 15 minutes.
Cool in the pan for 15 minutes. Lift out and cool completely on a rack before slicing. The swirl shows off best once it’s fully set - a tiny bit of patience goes a long way.