Line 5-by 4-inch disposable aluminum loaf pan with plastic wrap, allowing excess to hang over sides.
Place water in small bowl, sprinkle gelatin over top, and let mixture sit for 5 minutes. Pulse cheese, milk powder, salt, and cream of tartar in food processor until combined, about 3 pulses.
Meanwhile, bring milk to boil in small saucepan. Off heat, stir in softened gelatin until dissolved, and transfer mixture to 1-cup liquid measuring cup. With processor running, slowly add hot milk mixture to cheese mixture until smooth, about 1 minute, scraping down bowl as needed.
4. Immediately transfer cheese mixture to prepared pan, pressing to compact. Wrap tightly and chill at least 3 hours, or overnight.