Place cookies on a flat surface. Prepare frosting by mixing powdered sugar, butter, milk and vanilla with electric mixer until light and fluffy. Add more milk, one drop at a time, until desired consistency for piping. Add food coloring, one drop at a time and mix until desired color is reached.
Place frosting in a sandwich size ziplock bag and cut small tip out of corner. Seal bag and squeeze on top of cookies to resemble flip flops. Let air dry 30 minutes to 1 hour. Serve.