1. Place a cookie cutter, Mason jar ring, or a ring made from aluminum foil inside the slow cooker. Butter a 5-inch mini loaf pan and set aside.
2. Whisk together the flour, baking soda, and salt in a small bowl. Add oil, brown sugar, egg yolk, banana, milk, and vanilla, stirring just until flour mixture is incorporated. Fold in nuts.
3. Scrape batter into prepared loaf pan. Transfer loaf pan to sit on top of ring in slow cooker.
4. Place paper towels, still attached together, folded as necessary, over the top of the slow cooker. Cover and cook on High for 2 hours. When a toothpick inserted in the middle of the loaf comes out clean, the bread is cooked. If it does not emerge clean, continue cooking an additional 30 minutes.
5. Carefully remove the loaf pan from the slow cooker. Run a knife around the inside of the loaf pan and turn the loaf out onto a rack to cool. Serve warm or at room temperature.
The paper towels prevent the natural build-up of steam from potentially dripping on the loaf and making the bread soggy