Begin by preparing the vegetables. Cut the tough green ends and the root ends off of the leeks, then slice vertically from the green part down into the white part, about 2 inches. Wash in running water, fanning the green tops out, until all dirt and sand are removed. Chop the cleaned leeks.
Remove the leaves from the cauliflower, and rinse under running water. Slice the head in two and then slice off the florets from the central stalk. Place the florets in a pot of cold water and heat just until the water comes to a full boil. Remove from heat and immediately drain, then rinse with cold water. Set aside about 1 cup of cauliflower for the decoration, breaking them into small florets.
In a large Dutch oven, or similar heavy soup pot, melt the butter, then add the chopped leeks. Cook, stirring occasionally for about 5 minutes, or until the leek starts to soften (without browning).
Add the flour and stir to coat the leeks completely. Add the stock or bouillon gradually, stirring continually with a wooden spoon to keep things smooth and blended (start very slowly and gradually add more as things blend). Bring the mixture to a boil, add the cauliflower, then adjust the heat so the soup is at a simmer (very low boil). Simmer for 30 minutes, or until the cauliflower is very tender.
Allow the soup to cool some, then process it in a blender, working in small batches and returning the soup to another pot. If you wish, you can strain the blended soup through a chinois for a very refined and smooth effect.
In a mixing bowl, whisk together the egg yolks and the heavy cream. Add one ladle full of warm soup and whisk to combine. Pour this mixture into the soup and warm on low heat to desired temperature. Do no boil the soup or the egg yolks will curdle. Season to taste with salt and pepper.
To serve place a couple of the reserved florets in each bowl, before pouring the soup on top. You can further decorate with paprika and chives if you wish, or you may like to include a few homemade croutons.
Makes 6 servings.