Steam potato slices, covered, 12 minutes or just until tender. Drain; let cool.
Combine the yogurt and sour cream in a bowl, and stir well. Spoon the mixture onto several layers of heavy-duty paper towels, and spread to 1/2-inch thickness. Cover with additional paper towels, and let stand for 10 minutes. Scrape mixture into a pastry bag fitted with a fluted tip; set aside.
Combine the chives and parsley, and press 1/2 teaspoon over 1 side of each potato slice; pipe about 1/2 teaspoon yogurt mixture onto herb mixture. Top yogurt mixture with 1/8 teaspoon caviar. Serve immediately.