Cream the butter and sugar with an electric mixer until light and fluffy. Add the eggs beating well.
Add the dry ingredients to the butter mixture and mix until just combined.
Add the bananas, sour cream, vanilla, and nuts. Stir until just combined - avoid overmixing. Pour batter into a greased 9 x 5 x 3" bread pan (or other baking insert that fits your cooker).
Lay 4 to 5 layers of paper towels on top of the crock pot. Cover with the crockpot lid and bake on HIGH for 3 to 5 hours, or until a toothpick comes out clean when inserted in the middle of the bread (note: my crockpot took 4½ hours - but every unit is different). Let cool for 15 minutes on a rack before removing it from the pan.
Notes
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TOP TIPS: Using a silicone bread pan that is the size of your crockpot. I love using silicone pans in case the sizing is slightly off as they are flexible and can squeeze in a little better. Don't forget to cover with paper towels to prevent the moisture from going into the bread as it bakes.
VARIATIONS: Use walnuts or pecans, but they can be omitted if you'd rather not use them.
SERVING SUGGESTIONS: Slice and serve warm or cooled. Butter is always good on banana bread.