Melt butter over medium heat in a large, lidded pot. Add leeks and cook 7 minutes, until softened. Add chicken broth, potatoes and pears. Bring to a boil. Reduce heat to medium-low, cover and cook 25 minutes.
Using a handheld blender (or a countertop blender, in batches), puree soup until smooth. (If using countertop blender, return mixture to pot.) While still hot, stir in brown sugar and salt.
Ladle soup into 8 bowls. Garnish with a dollop of crème fraîche and sliced chives, if desired.
Save time and let flavors deepen: Make soup in advance and store, airtight, in fridge.
