In a large Dutch oven or heavy bottomed pot, add butter and melt over medium-low heat.
Add the leeks, salt and pepper and cook, stirring often, until the leeks are slightly caramelized and very tender, about 25 minutes.
As the leeks cook, add water 2 tablespoons at a time as needed to prevent the leeks from sticking to the bottom of the pan.
Remove from heat and let cool completely.
When ready to serve, place sour cream in a medium bowl and add cooled leeks. Stir until well combined.
Serve immediately, or chill for up to 24 hours before serving. Serve dip with crudités, crackers, chips, or toasted bread.
