Mix together the heavy cream with the milk kefir in a jar. The heavier the cream, the thicker your resulting sour cream will be. You can keep experimenting to get the consistency that you want with the cream you are using. If you want to end up with a thinner sour cream, you can also bring down the fat content of the cream by mixing it with some milk.
The cream should immediately begin to thicken. At first, it won't be sour, but you could use it in place of mascarpone in recipes.
Fermenting the cream
To sour the cream, it needs to be fermented. Cover the jar, and allow it to rest at room temperature. At first, allow it to ferment overnight.
In the morning, taste the kefir sour cream. If you want it to be sourer, leave it out to ferment longer.
Once you've achieved the level of sourness that you want, use it right away or store it in the fridge for later. Storing it in the fridge will slow down the fermentation process and keep it from getting sourer as quickly.
Video
Notes
How to use it
Kefir sour cream is very versatile. If you use it before it gets sour, it is almost like mascarpone, and can be used for desserts.It also makes a great garnish. You can also use it in any recipe that would normally call for sour cream. It goes especially well as a garnish for Mexican recipes or served atop baked potatoes and/or baked sweet potatoes.
How long does it keep?
Because it has been fermented, kefir sour cream will keep a long time. (It will keep much longer than the cream would have kept had it not been fermented.)Even outside of the fridge, it generally doesn't go bad, it just gets sourer and sourer with time. Eventually, you may find it too sour to enjoy. (It likely won't last that long anyway, as it's delicious!)