In the mixing bowl of a stand mixer, using the whisk attachment, add the butter, sugar, vanilla and egg, and blend until light and fluffy.
Add the mashed banana, and blend.
In a separate bowl, add the flour and baking powder, and stir well to blend.
Add the flour a little at a time and mix on low to blend until all the flour is added. Remove the whisk attachment, and use the paddle. Continue to blend the cookie dough until it's completely blended and a smooth ball starts to form. Scrape down the mixing bowl as needed.
Line two cookie sheets with parchment paper, and roll the cookie dough into small balls, about 1 inch balls, and place the balls on the parchment paper. Bake at 350 degrees for about 15 minutes, or just until the cookies start to turn a light golden brown around the edges. About half way through baking the cookies, check the oven, and the cookies may have started to crack a little. Take a flat bottom glass, dip it in sugar before lightly applying to each cookie - smash the cookies very gently and slightly, making them a little flatter, and spread out a little more. This will give the cookies the crinkle look.
When the cookies are done, remove from the oven to a wire rack to cool completely.
To make the frosting
In the mixing bowl of a stand mixer, using the whisk attachment, add the softened butter and cream cheese and blend to combine.
Add the powdered sugar a little at a time, blending after each addition, until the frosting is smooth and fluffy.
Divide the frosting evenly into two bowls. In one bowl, add a couple drops of the yellow food coloring gel, and stir well to blend. If the frosting thinned a bit add a tablespoon of powdered sugar, and stir to blend. The other bowl of frosting remains white, to swirl with the yellow.
To decorate the cookies
Using plastic wrap tear off a piece about 10 inches wide and place the frosting length-wise in the middle of the plastic wrap; it will look like two long ribbons of frosting.
Then, roll the plastic wrap around the frosting, making sure to keep it long, or length-wise like a long ribbon.
Snip the end off of the plastic wrap, on one end only, and place it in a pastry bag with the large star tip.
Pipe the Frosting onto the cookies, using a swirl, and bring it up to a point. Place a Banana slice on the top of each cookie. Allow the Frosting to set or firm up a bit, about 30 minutes.