Preheat the oven to 350 degrees.

In a large bowl cream together the butter and brown sugar. Add in the egg and vanilla and mix well.

Mash the banana and fold in the banana and the sour cream.

Add in the flour, cornstarch, baking soda, cinnamon, and salt. Mix until just combined.

Scoop out 1/4 cup of dough and place on a baking sheet at least 3 inches apart.

Bake for 11-13 minutes. Remove from oven and let them cool for 10 minutes on the baking sheet before removing to a cooling rack.

To make the frosting place the cream cheese and butter in a large bowl. Mix on medium speed with a mixer until the mixture is smooth. Add in the powdered sugar and vanilla and mix until well combined. Add in enough milk to reach your desired consistency and mix until smooth.

To make the brown sugar bits combine all ingredients in a small bowl and mix until a crumble forms.

To finish the cookies place 1-2 tablespoons of frosting on top of the cookies and swirl it on top with a spoon. Sprinkle with brown sugar bits.