Preheat your oven to 350°F. Grease and flour a 9x5-inch loaf pan, then set aside.

In a large bowl, melt the butter and let it cool slightly. Whisk in the sugar, then the eggs and vanilla until smooth. Gently stir in the flour and baking soda, then fold in the mashed bananas just until combined. Do not overmix.

In a small bowl, use a fork to stir together the melted butter, granulated sugar, dark brown sugar, and cinnamon until thick and well combined. If the mixture seems too thick to swirl, add a little more melted butter, a teaspoon at a time, until it reaches a spreadable consistency. If it is too thin, it won't hold its shape during baking.

Spread about 1 cup (1/4 of the batter) evenly into the bottom of the prepared pan. Dollop about 1 tablespoon (1/4 of the cinnamon mixture) over the batter, then use a table knife or the back of a wooden spoon to create thick swirls.

Add another 1 cup (1/3 of the remaining batter) evenly over the first layer, then dollop another 1 tablespoon (1/3 of the remaining cinnamon mixture) and swirl again.

Repeat with another 1 cup (1/2 of the remaining batter) and 1 tablespoon (1/2 of the remaining cinnamon mixture), swirling well.

Spread the remaining batter evenly over the top, dollop with the last of the cinnamon mixture, and make large swirls throughout the entire loaf. Arrange the banana slices in a line down the center.

Place the loaf pan on a baking sheet in the middle rack of the oven and bake for 60–70 minutes, until golden brown and a toothpick inserted in the center comes out clean. If the top browns too quickly, tent loosely with foil and continue baking until the center is fully cooked.

Let the bread cool in the pan for at least 30 minutes, then turn it out onto a wire rack to cool completely before slicing.