In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2-3 minutes using a hand mixer or whisk.

Beat in the eggs one at a time, ensuring each is fully incorporated, then stir in the vanilla extract. Scrape down the sides of the bowl.

In a separate bowl, mash the 4 ripe bananas until mostly smooth with a few lumps remaining. Add the mashed bananas to the butter mixture and mix on low speed or by hand until combined.

In another bowl, whisk together the sifted flour, baking soda, salt, and optional cinnamon.

Gradually add the dry ingredients to the wet mixture, alternating with the milk, mixing until just combined. Avoid over-mixing to keep the bread tender.

Grease the inside of your slow cooker with butter or use a slow cooker liner for easy removal. Pour the batter into the slow cooker and spread it evenly.

Cover and cook on HIGH for 1.5 to 2 hours, or until a toothpick inserted in the center comes out clean. Check after 1.5 hours to avoid overcooking.

Turn off the slow cooker and let the banana bread cool in the cooker for 10-15 minutes. Then carefully remove and place on a wire rack to cool completely before slicing.