Grease the inside of your slow cooker insert with butter or non-stick spray. If desired, line the bottom with parchment paper for easy removal.

In a small bowl, whisk together the 1/4 cup granulated sugar and 2 teaspoons ground cinnamon. Set aside.

In a large mixing bowl, beat the softened butter and 1 cup granulated sugar until light and fluffy. Add the eggs one at a time, beating well after each, then mix in the vanilla extract.

Fold in the mashed bananas and sour cream until just combined. Do not overmix.

In a separate bowl, whisk together the flour, baking soda, salt, and cream of tartar.

Slowly add the dry ingredients to the wet ingredients, mixing until just combined. Spoon half the batter into the prepared slow cooker. Sprinkle 2/3 of the cinnamon sugar mixture evenly over the batter and gently swirl it in with a knife. Top with remaining batter and sprinkle the rest of the cinnamon sugar on top (do not swirl this layer).

Cover the slow cooker and cook on LOW for 3 hours, or until a toothpick inserted in the center comes out clean (a few moist crumbs are fine). Avoid lifting the lid during cooking to maintain even heat.

Turn off the slow cooker, remove the lid, and let the bread cool in the insert for 30 minutes. Then, carefully remove the bread (using parchment if lined) and transfer to a wire rack to cool completely before slicing.