Preheat oven to 350ºF. In a medium-sized bowl, whisk together the cake flour, baking powder, baking soda, cornstarch, and salt. Set aside.

In a large bowl, use a stand or handheld electric mixer on medium speed to beat the butter for 2 minutes, until creamy. Add the sugar and beat on medium speed for two more minutes. Add the sour cream, egg, and vanilla and whisk until combined. Add the dry ingredients and fold in using a spatula.

Refrigerate the cookie dough for at least 30 minutes. Then, use a spoon to place the cookie dough onto a baking sheet (The dough will be sticky!). Bake for 12-13 minutes at 350ºF. Allow the cookies to cool completely before frosting.

Make the frosting while the cookies are cooling. In a large bowl, use an electric handheld or stand mixer on medium speed to beat the butter for 2 minutes, until smooth and creamy. 

Add the heavy cream, vanilla extract, and powdered sugar and beat on low speed until combined (If the frosting seems too thick, add more heavy cream one teaspoon at a time). If desired, add the gel food coloring a few drops at a time, mixing on low speed. Allow the cookies to cool completely before frosting.