Crush fennel seeds using a spice grinder or mortar and pestle until smooth.

Cut leeks in half lengthwise and then cross-wise into 1/2 inch pieces. You want about 4 cups of chopped leeks (a bit more or less is fine). Soak leeks in a large bowl of cold water to get rid of dirt and grit. Lift them out of the bowl using your hands and into a colander leaving the dirt behind at the bottom of the bowl. Rinse leeks in the colander under cold water.

Heat butter and olive oil in a large pot (4 to 5 quarts) over medium heat. Add leeks and potato, and cook until leeks become vivid green and begin to wilt, about 5 minutes. Add wine or vermouth, chicken broth, bay leaf, tarragon, fennel seeds, cayenne pepper, salt, and black pepper. Bring soup to a boil, lower to a simmer, and cook until the potatoes are tender, about a 30-40 minutes.

Remove the bay leaf. (Don't forget!)

Using a Vitamix, immersion hand blender, regular blender or food processor, purée the soup in batches until smooth. Return the soup to the pot, add cream, and bring to a simmer. Season with salt and pepper.

Garnish each serving with a dollop of creme fraiche and sprinkling of crumbled bacon, if desired.