Prep - Soften your butter and cream cheese to room temp. Line cookie sheets with silicone liners, parchment paper, or cooking spray. Preheat your oven to 350 degrees.
Cookie Dough
In a large bowl, combine the butter and sugars. Beat for 1-2 minutes with an electric mixer until fluffy and creamy.
Mash the bananas and add them to the butter mixture with the egg and vanilla. Beat on low speed just until combined.
Whisk together the flour, baking powder, soda, salt, cinnamon, and nutmeg. Stir into the wet ingredients.
Scoop about 1 1/2 tablespoons of cookie dough onto a prepared cookie sheet, leaving 2-3 inches between each cookie.
Bake at 350° for 8-9 minutes, or until edges are starting to turn golden brown and the cookies are set in the middle. Let cool on the pan for 10 minutes, then transfer to a wire rack to cool completely.
When cookies are completely cool, spread frosting on each one. If desired, sprinkle the tops with chopped nuts or a sprinkle of nutmeg.
Cream Cheese Frosting
Beat the butter and cream cheese until smooth. Add the powdered sugar, vanilla, and salt. Beat until smooth and creamy. You can add milk if you need to, but I don't usually need it.
Notes
For best results, use over-ripe bananas. You want them to be completely covered in brown spots or turning black.
I like to use a 1 1/2 tablespoon cookie scoop to portion the dough. The cookies bake more evenly when they are all the same size.
Leftover cookies can be stored in an airtight container at room temperature for 2-3 days, or in the refrigerator for 4-5 days. Place waxed paper between layers.