Preheat your oven to 350°F. Grease an 8×8-inch baking pan with a small amount of ghee, butter, or oil. In a medium bowl, whisk together the dry ingredients: flour, cinnamon, ginger, nutmeg, baking soda, baking powder, and salt. In a separate medium to large bowl, mash the banana until smooth. If necessary, melt the coconut oil. Add the egg, milk, melted oil, maple syrup, and vanilla extract to the mashed banana and mix thoroughly to combine. Gradually add the dry ingredients to the wet mixture and stir well. Fold in the shredded carrots, raisins, and walnuts. Pour the batter into the prepared pan and bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out clean. Allow the bars to cool completely. While baking, prepare the frosting and place it in the refrigerator to chill until the bars are cool. Once the bars have cooled, spread the frosting evenly over the top. Optionally, sprinkle a bit of cinnamon on top. Refrigerate the frosted bars for 10 minutes to set. Slice into 9 pieces and enjoy!