You can make this with cow milk, of course. If it's too hot to click, just substitute buttermilk for the kefir and greek yogurt for the yogurt. I'm currently trying goat milk because cow milk is giving me sinus issues. Goat milk tastes mildly like goat cheese, and tastes really good with fruit.
[ETA: Goat milk kefir and yogurt come in quarts, might as well just double the recipe; so you have goatskal for two weeks, which is no bad thing.]
4 egg yolks
1/4 cup sugar
1 tablespoon + 1 teaspoon vanilla
1/2 cup lemon juice or juice of two lemons
1 quart goat milk kefir
1 quart goat milk yogurt
Whisk the egg yolks in a large bowl or pitcher with the sugar, then whisk in the vanilla and lemon juice, then whisk in the kefir and the yogurt.
Serve with fruit:
