Preheat your crockpot on HIGH for 10 minutes, then add a small trivet or crumple parchment paper at the bottom to prevent sticking, and grease the sides lightly with butter.

In a large mixing bowl, mash the ripe bananas until mostly smooth, leaving a few chunks for texture.

Add the melted butter, brown sugar, granulated sugar, egg, vanilla extract, and buttermilk to the mashed bananas. Whisk vigorously until the mixture is uniform and shiny.

In a separate bowl, whisk together the flour, baking soda, ground cinnamon, and salt.

Gradually fold the dry ingredients into the wet mixture. Stir just until combined—do not overmix.

In a small bowl, mix the brown sugar, flour, cinnamon, and melted butter until it forms a thick paste.

Pour half of the batter into the prepared crockpot. Drop spoonfuls of the cinnamon swirl paste over the batter, then gently swirl with a knife in a figure-eight pattern. Pour the remaining batter on top.

Cover the crockpot (place a clean kitchen towel under the lid to absorb excess moisture), and cook on LOW for 3 hours or on HIGH for 1.5 hours, until a toothpick inserted in the center (avoiding the swirl) comes out clean. Note: Cooking times may vary by crockpot; start checking after 1.5 hours on high.

Turn off the crockpot and remove the lid, but leave the bread inside for 10 minutes to set. Carefully lift out using the parchment and cool on a wire rack for another 10 minutes before slicing.