1. Combine skim milk, buttermilk, rennet, and salt in large stainless steel or glass bowl. Cover lightly with plastic wrap, and let sit at room temperature for 18 to 24 hours.
2. Curd will separate from whey and float to top of bowl. Use slotted spoon to remove whey and spoon into molds.
3. Fit wire rack in bottom of roasting pan, and place molds on rack to drain. Cover with plastic wrap and let rest in refrigerator for 6 to 8 hours.
4. Remove cheese from molds, and store in tightly sealed containers in refrigerator for up to 2 weeks.