Thick buttermilk makes a great base for a creamy dressing with less fat and tons of flavor.
In a large pot of salted boiling water, cook pasta 1 minute less than package instructions; add fennel and edamame and cook 1 minute. Drain and let cool slightly.
Meanwhile, in a large bowl, whisk together buttermilk, mayonnaise, lemon juice, and basil. Season with salt and pepper. Add pasta mixture, watercress, and fennel fronds to bowl and toss to combine. Season with salt and pepper.