Infuse vinegar with dill. Combine vinegar and 1 tablespoon minced fresh dill in a small saucepan over low heat. Heat just to a simmer then remove from heat and let cool.
¼ cup white vinegar, 3 tablespoons fresh dill
Meanwhile, place potatoes and 1 tablespoon salt in a large saucepan and cover with cold water. Place the ½ cup chopped dill stems inside cheesecloth and secure to side of saucepan so that the dill is immersed in water. Bring to boil over high heat then reduce and simmer just until potatoes are fork-tender 12-15 minutes.
1 ½ pounds baby potatoes
Discard dill packet and drain potatoes then place in a large bowl. Drizzle 2 tablespoons of the dill vinegar over warm potatoes and toss gently to coat. Refrigerate until cool, at least 30 minutes.
Meanwhile, make creamy dill dressing. Whisk sour cream, mayo, dijon mustard and remaining dill vinegar with salt and pepper.
½ cup sour cream, ¼ cup mayonnaise, 1 tablespoon Dijon mustard, ½ teaspoon salt, ¼ teaspoon black pepper
Once cool, toss potatoes with creamy dressing, gently smashing a few of the larger potatoes for texture. Stir in scallions and remaining 2 tablespoons chopped dill.