Prep. Heat oven to 350F. Grease and line a 9x5 inch loaf pan with parchment (leave overhang for easy lifting).
Make cheesecake swirl. In a bowl, beat cream cheese and sugar until smooth. Mix in egg yolk, vanilla, and flour (if using). Set aside.
Mix wet ingredients. In a large bowl, whisk melted butter, brown sugar, and granulated sugar. Whisk in eggs, vanilla, mashed bananas, and yogurt until combined.
Add dry ingredients. Sprinkle flour, baking soda, salt, and cinnamon over the wet mixture. Stir gently just until no dry flour remains. Fold in nuts or chocolate chips if using.
Layer. Pour about half the banana batter into the loaf pan. Spoon half the cheesecake mixture over the top. Repeat with remaining banana batter and cheesecake mixture.
Swirl. Use a butter knife to make a few gentle figure-eight swirls through the batter (don’t overdo it).
Bake. Bake 50 to 65 minutes, until the loaf is set and a toothpick inserted into the banana bread part comes out mostly clean (a few moist crumbs are okay). If the top browns too fast, tent loosely with foil for the last 10 to 15 minutes.
Cool. Cool in the pan 15 minutes, then lift out and cool completely on a rack before slicing.
