Preheat oven to 350 degrees. Line 9x13 inch baking pan with parchment paper. Set aside.
In a large bowl, beat butter and cream cheese on high until smooth. Add granulated sugar, egg and vanilla and beat until smooth.
In a separate mixing bowl, sift flour and salt.
Gradually add flour mixture to wet ingredients, mixing on low until combined.
Pour dough into prepared baking pan and transfer to oven for 20-25 minutes. Do not over bake - this bars should remain slightly under baked in the middle. Remove from pan and transfer to cooling rack. Cool completely.
To make the frosting, combine the butter and cream cheese and beat with an electric mixer until well smooth. Slow add confectioners' sugar ½ cup at a time. Add 1 tablespoon of milk and 1 teaspoon of vanilla and food colouring and continue to beat. Add milk, 1 tablespoon at a time, as needed until desired consistency is achieved. Continue to beat frosting from 2-3 minutes until light and fluffy.
Spread frosting evenly onto cookie bar base. Top with sprinkles. Cut into 20 bars. Serve and enjoy!