Combine 7 parts heavy whipping cream with 1 part buttermilk.
Whisk and cover with cheesecloth.
Allow to stand at room temperature overnight.
The following day, whisk again and then refrigerate until ready to use.
In 8 quarts of heavily salted water with thyme, bay leaf and peppercorns, start potatoes in cold water and bring up to a boil.
Immediately reduce to a simmer and, for approximately 8 to 10 minutes, cook until fork-tender.
Carefully remove from water and allow to cool on a sheet tray in the refrigerator. This can be done a day ahead.
Slice the potatoes uniformly and pan-fry.
Remove from oil onto paper towel and immediately season with salt.
Build concentric rings of potatoes that grow progressively smaller, topping each layer with small amounts of dill, crème fraiche and roe.
When the tower is complete, add another layer of dill, crème fraiche and roe, then a small amount of black salt.
Serve immediately. The hot potatoes will begin melting the crème fraiche immediately.