Do not use ultra-pasteurized cream for this recipe, or it will not clot. You can use heavy whipping cream that has 'pasteurized' printed on the carton. It may take longer to clot than raw cream but is an acceptable substitute.
Raw (unpasteurized) cream is recommended if you can get it. State regulations vary on the sale of raw milk products. Check your local farmers' market for dairy purveyors. We do not recommend consuming raw dairy products, however, the cream is safe to eat once cooked.