In a clean, non-reactive bowl or container, such as a mason jar, pour the heavy cream and add the buttermilk. Mix gently with a spoon until evenly distributed, ensuring that the buttermilk is fully incorporated into the cream.

Cover the bowl with plastic wrap, or use a lid if you're using a mason jar. Let it sit undisturbed at room temperature away from sunlight, such as in an off oven, for 24 hours.

Use a whisk to gently break up any lumps of fat on top and mix in the salt. Whisk just until the lumps are resolved and the salt is evenly distributed.

Cover the container and refrigerate it for at least 2 hours. The crema Mexicana will continue to thicken as it chills. For the best results, let it sit overnight.

After that time, the crema Mexicana will be luscious and thick with full body, yet still pourable. Mix it with a spoon to loosen it up a bit more. Store in a non-reactive, airtight container in the refrigerator.