2/3 cup walnuts, toasted and chopped
1/2 cup packed brown sugar
1/4 cup white sugar
8 tablespoons (1 stick) unsalted butter, softened
2 large eggs
1 teaspoon vanilla extract
1 cup all-purpose flour
1 cup whole wheat flour
2 teaspoons baking powder
1 teaspoon cinnamon
1/2 teaspoon salt
1/2 cup milk
2 cups chopped bananas, from about 3 ripe bananas
Heat the oven to 400°F. Spray the top of your muffin tin with non-stick coating or place muffin cups in all the wells.
Scatter the walnuts on a baking sheet and toast in the oven until slightly darkened and very fragrant, 8 to 10 minutes. Transfer them to a cutting board and roughly chop while still warm. Set aside.
In a mixer on high speed, cream together the brown sugar and white sugar with the softened butter until the mixture resembles fluffy frosting, about 1 minute. Mix in the eggs one at a time until fully incorporated, about 1 minute each. Mix in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, whole wheat flour, baking powder, cinnamon, and salt. With the mixer on low speed, mix a third of the flour mixture into the butter-sugar mixture followed by a third of the milk. Continue alternating between the flour and the milk, mixing just until the flour is incorporated. It's ok if there is still some flour on the sides of the bowl. Do not over-mix.
Scrape the sides of the bowl with a spatula and stir gently to incorporate the last of the flour. Add the chopped bananas and nuts, and gently fold them into the batter.
Divide the batter between the muffin cups. The batter should fill the cups and mound slightly on top. (If you prefer smaller muffins, fill the cups only three-quarters full and bake in two separate batches.)
Place the muffin tin in the oven. Bake for 20 to 25 minutes, rotating the pan once in the middle of baking. The muffins are done with the tops looked cracked and toasted, and when a toothpick inserted in the middle of one of the muffins comes out clean.
Let the muffins cool enough to handle, then transfer them to a wire rack to finish cooling. Leftover muffins can be kept in an airtight container at room temperature for 3 to 4 days or frozen for up to 3 months. (Reheat frozen muffins in an oven or toaster oven at 300° or for a few minutes in the microwave)