Add 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 lime juice, salt and mix until well combined.
Serve immediately or cover and store in the fridge for up to 2 weeks.
Notes
*I let mine sit for 24 hours on the top of the refrigerator and it was beautifully thick the following day. However, I made mine in the winter when the house is on the colder side. If you make this in the summer, you may only need to let it sit out for 12-18 hours.The crema tends to thicken up the longer it sits in the fridge. If it becomes too thick for your liking, you can thin it out by mixing in a splash of lime juice before serving.