Add the heavy cream and buttermilk to a small bowl or glass jar. Stir together to combine.
Cover tightly with plastic wrap or a lid and let sit in a warm spot for 12 to 24 hours*.
Add the lime juice and salt. Mix until well combined.
Serve immediately or cover and store in the fridge for up to 2 weeks.
Notes
Resting time. I left out the heavy cream and buttermilk mixture on the counter at room temperature for 12 hours before mixing salt and lime juice. You can also allow the crema to rest in the fridge for 24 hours to allow it to thicken.
Thickening texture. As the crema sits in the fridge after preparing, it can continue to thicken. To thin it out before serving, you can add a splash of lime juice or buttermilk and whisk with a fork until it reaches your desired consistency.
Add some spice. Make a chili lime crema by adding 1 teaspoon of chili powder when mixing in the lime juice and salt.
Add some heat. Make it spicy by adding 1 teaspoon of chipotle powder or ½ teaspoon cayenne pepper.