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Preheat oven to 350F. Line 2 12-cup muffin tins with liners.
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In the bowl of a stand mixer fitted with a paddle attachment, cream butter and sugar together. Add eggs, one at a time, and banana and beat until mostly smooth.
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In a separate bowl, whish together flour, baking powder, baking soda, cinnamon, and salt to remove any lumps. Add this to the butter mixture and beat until just combined. Use a rubber spatula to fold in the walnuts.
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Divide the batter among the 24 muffin cups. Bake 22 - 24 minutes or until a toothpick inserted into the center comes out clean. Transfer to a wire reack to cool completely.