Place a cookie cutter, Mason jar ring, or a ring made fromaluminum foil inside the slow cooker. Butter a5-inch mini loaf pan and set aside.
Whisk together the flour, baking soda, and salt in a smallbowl.
Add oil, brown sugar, egg yolk, banana, milk, andvanilla, stirring just until flour mixture is incorporated. Foldin nuts.
Scrape batter into prepared loaf pan.
Transfer loaf pan tosit on top of ring in slow cooker.
Place paper towels, still attached together, folded asnecessary, over the top of the slow cooker. Cover andcook on High for 2 hours. When a toothpick inserted in themiddle of the loaf comes out clean, the bread is cooked. Ifit does not emerge clean, continue cooking an additional 30minutes.
Carefully remove the loaf pan from the slow cooker. Run aknife around the inside of the loaf pan and turn the loaf outonto a rack to cool.