Cream Of Fiddlehead Soup

• 2 french shallots (finely chopped)
• 2 egg yolks
• 1 1/2 fiddleheads see note
• 1,900 ml chicken stock
• 240 ml cream (heavy)
• 45 gr carrot (finely diced)
• 22 gr leeks thinly (sliced)
• 14 gr butter