Carve an eye-catching serving bowl for this dip (and any others): Cut about a 4-inch-diameter round out of the top of a head of cabbage using a paring knife, then scrape out the center using a large metal spoon.
Melt butter in a large skillet over medium-low heat. Cook leeks, stirring often, until caramelized, about 35 minutes. Transfer to a bowl and let cool.
Stir in sour cream, goat cheese, bacon, and 1 1/2 teaspoons salt.
Dip can be refrigerated for up to 1 day. Serving idea: Serve dip with assorted crudites and breadsticks.