Delicata Leek Soup with Fennel

3 medium sized potatoes, (peeling optional)
1 Delicata squash
water for boiling
1/4 cup butter
3 leeks, chopped
1 fennel bulb, chopped
1 tsp salt
1/2 cup buttermilk
1 tbsp chopped fresh fennel leaves, plus more for garnish

Chop potatoes and squash into fourths. Bring water to boil, add potatoes and squash, and reduce to a simmer. Melt butter in skillet. Add leeks and fennel bulb and saute until soft, about 10 minutes. When potatoes and squash are soft (about 15-20 minutes), reserve 2 cups of the water used for boiling and drain the rest. Remove the squash pieces from the pot, wait until they are cool enough to handle, and peel the skins off (they should come off very easily, if not, boil them longer). Place the leek mixture and squash and potatoes plus reserved water in blender (may have to do this in two or more batches). Puree until smooth. Add salt, buttermilk, and fennel leaves, and blend for a minute longer. Pour back into pan and reheat, if necessary. Otherwise, serve hot, topping with more fennel leaves.