Preheat oven to 200ºC (180ºC fan) and grease a 23cm round springform tin.
Cream the butter and sugars together until light and fluffy (about 5 mins). Beat in the eggs and vanilla. Fold in the flour and salt with a large metal spoon until combined, then fold in the chocolate chips.
Tip into prepared tin and spread to level. Bake for 30 mins. Allow to cool completely in the tin.
Make the buttercream: beat butter and sugar together until smooth then beat in vanilla and milk. Divide the mixture into as many colours as you have, leaving one white. Pipe your desired message onto your cookie and decorate.
Recipe by 5* trending recipes with videos at https://food.theffeed.com/2021/02/01/millies-cookie-recipe/