In a deep saucepan over medium-high heat, saute leeks in butter just until tender, about 5 minutes. Add white potatoes, sweet potatoes, and chicken broth. Cover and bring to a boil. Reduce heat to simmer; cook 10 minutes or until potatoes are tender when pierced with a sharp knife. Remove from heat and uncover. Cool about 30 minutes at room temperature.
Place one-fourth of mixture in a food processor. Puree 1 minute. Using a wooden spoon, press through a sieve to remove any lumps. Repeat until all potatoes are pureed. Cover; chill 2 hours or overnight.
To serve, ladle 2 cups of potato mixture into a large bowl. Whisk in 1 cup milk. Swirl ketchup on kids' servings if desired.
