Line the bottom of a strainer with cheesecloth, a coffee filter, or paper towels and set over a medium bowl (strainer should not touch bottom of bowl). Spoon in yogurt, cover, and refrigerate 8 hours or overnight, until yogurt cheese is thich and creamy.
Remove seeds and ribs from jalapeños with mellon baller (wear gloves when handling, as the peppers can burn); mince. Mix jalapeños, tomatoes, onion, cilantro, and salt in large bowl. Fold in yogurt cheese and sour cream.
Halve, pit, and peel avacados. Mash with potato masher; sprinkle with lime juice. Quickly fold into tomato mixture. Makes 3 cups guacaomle. Serve wth tortilla chips. To keep a lid on the fat, serve baked, not fried, tortilla chips.
For an even greater kick, add 1 small diced serrano pepper instead of the jalapeño.