Once cake is cool, start making icing.
In a large bowl, or in a stand mixer, add butter and mix with the paddle attachment. Mix on high speed until the butter has creamed, occasionally scraping down the sides (3-4mins).
Reduce speed to low and gradually add the powdered sugar, scrape down sides to make sure mixture is combined well.
Slowly add milk. The icing might look like it is separating, continue mixing and increase speed and it will come back together.
Scrape down sides and add vanilla, food coloring, and salt. Increase the speed to high until the frosting looks smooth. Reduce speed to low and mix for 5 mins. The reduced speed will help eliminate air bubbles.
Once done it is ready. Spread on cake and serve. Use any saved cookies to decorate.
The cake is best stored in the fridge in an airtight container for up to 3-4 days. Let come to room temperature when serving.