Step 1Press the yolks through a sieve into a small food processor. Add the mayonnaise, vinegar, crème fraîche, and mustard and process until smooth, scraping down the sides as necessary. Season with 3/8 teaspoon salt, or to taste. This makes about 1 ½ cups filling.
Step 2For really pretty eggs, feed the mix into a pastry bag (alternatively, you can jerry-rig one with a large resealable plastic bag; snip off a corner before piping). Pop it into the fridge for 30 minutes. Put the egg whites on a plate, cover with plastic wrap, and put them in the fridge as well.
Step 3Pat the whites dry with a kitchen towel and pipe an equal amount of the yolk mixture into each white. Top each one off with a sprinkle of chives and chervil and a dusting of cayenne or paprika. If you like, add a sprinkle of sea salt and a drizzle of olive oil and serve.