Preheat oven to 400 degrees F and grease muffin tins with butter.
In a large bowl, sift together flour, baking power, baking soda, pumpkin pie spice, sugar, and salt.
In a medium sized pan over medium heat, melt the butter. Swirl occasionally to ensure butter is cooking evenly. As the butter melts, it will begin to foam and the color will get darker. Scrape bottom of pan with wooden spoon to release any brown specs. Once you start smelling the nutty aroma, remove browned butter from heat and allow to cool.
In a separate bowl, whisk eggs, buttermilk, yogurt, vanilla, and mostly cooled browned butter together. Make a well in the dry ingredients and pour in the liquid ingredients. Fold together and do not over mix. Fold in the mashed banana and walnuts.
Spoon batter into greased muffin tins and bake in preheated oven until golden brown, about 20-30 minutes (mine took longer because I used a stone muffin pan). When done cooking, immediately remove from muffin tins and allow to cool on cooling rack.
Notes
Use a cookie scoop to easily distribute the batter to the muffin tin wells
Sour cream can be used as a substitute for buttermilk and/or yogurt, if needed
You can omit the walnuts, but I absolutely love them in the muffins