Toast the poppy seeds lightly in a small skillet over medium heat for 1 to 2 minutes. Transfer to a cool plate.
In a small bowl, combine the crème fraîche, yogurt, chopped tarragon, lemon juice, garlic, and poppy seeds. Stir in 1/2 to 1 Tbs. water to thin the mixture to a creamy salad-dressing consistency. Season with cayenne and salt to taste.
Just before serving, toss the butter lettuce in a large bowl with just enough of the dressing to lightly coat the leaves (you'll have leftover dressing). Season with salt and pepper to taste. Arrange the salad on individual serving plates, scatter the whole tarragon leaves on the top, and serve.
Photo: Scott Phillips