Bring a medium pot of water to a simmer. Add the vinegar. Gently break the quail eggs, one by one, into the simmering water and poach for 1 minute. Remove the eggs from the water with a slotted spoon and place in a bowl of cold water to stop the cooking. Place one cooled, poached quail egg on each crouton. Top with a little hollandaise sauce and caviar.
Beat egg yolk. Place bowl over a pot of simmering water, making sure that the bottom of the bowl does not touch the water. Beat the egg until thickened, pale yellow and doubled in volume. Remove the bowl from the heat. While beating, slowly add the butter in a slow, but steady stream. When the butter is incorporated, add the lemon juice and Tabasco sauce. Mix well. Season to taste with salt and pepper. Keep warm.