Combine flour, brown sugar, baking powder, baking soda, salt, and1-1/2 teaspoons cinnamon in a large mixing bowl.
Cut butter into small pieces, and add to flour mixture.
Using a pastry blender or two forks, mix until butter is the size of small peas.
In a large mixing cup combine buttermilk, egg, and vanilla.
Slowly add half of the liquids; add all of the mashed banana and walnuts.
Stir to combine.
If mixture is too dry, add more of the liquids. (The dough should form a ball.)
Place bowl in refrigerator, and preheat oven to 400.
Once oven is heated, place dough on a lightly floured surface and press into an 8-inch round.
Cut into 8 wedges. (This will be a moist dough; do not panic!)
Transfer wedges to rimmed cookie sheet lined with parchment paper.
Bake for 15-20 minutes, or until bottoms are golden brown.
Transfer to cooling rack.
While they cool, combine 1 cup powdered sugar, 2 tablespoons softened butter, 1 tablespoon milk, and 1/2 teaspoon cinnamon.
Stir until combined, adding more sugar for sweetness and more milk to get the desired consistency.
Frost scones, and enjoy!