Preheat oven to 350F. Line muffin tin with paper liners and set aside.
In a large bowl or the bowl of a stand mixer, cream butter together with brown and granulated sugar until light and fluffy.
Mix in bananas. Blend in eggs, milk, and vanilla. There may be lumps from the bananas.
Add flour, baking soda, baking powder, cinnamon, and allspice. Mix until just combined.
Coat walnuts in 1 teaspoon flour and fold into batter.
Spoon the batter evenly between prepared muffin tin (I use a 3 Tablespoon cookie scoop).
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean. Cool for 5 minutes in muffin tin. Carefully remove and cool on a cooling rack or serve warm.
Notes
Bring butter and eggs to room temperature for faster and easier blending.
Make sure baking soda and baking powder are not expired for best results.
Toast nuts or seeds before using to intensify their flavor and crunch.
Aerate flour with a whisk before measuring it out.