Combine the sour cream and heavy cream in a bowl and whisk. Set in a warm place for 24 hours. Refrigerate
Add the potatoes to a pan and cover with water. Bring to a boil and cook until fork-tender, about 25 minutes. Drain the water and let cool. Cut each potato in half. Scoop out a little of the center with a spoon. Sprinkle with salt. Pipe the crème fraiche into the center of the potato and top with caviar