Crack the eggs into a large bowl.
Beat the eggs well with a mixer for a few minutes, until light and foamy.
In a separate large bowl, add the flour and baking soda.
Add the sugar to the flour mixture.
Pour in the oil and mix briefly until just combined.
Add the milk and whisk well until smooth.
Pour in the kefir and whisk again, making sure there are no lumps.
Finally, add the beaten eggs and mix until everything is fully incorporated.
You should now have a very liquid batter, even thinner than classic pancake batter.
Grease a deep baking dish or Dutch oven generously with soft butter.
Pour the batter into the prepared pan.
Wrap the pan tightly with aluminum foil over the top, underneath and around the sides. Bake in a preheated oven at 390°F (200°C) for 2 hours, then reduce to 210°F (100°C) and bake for another 2 hours. Do not open the oven while it bakes.
The black baba is ready when deeply browned, set but slightly jiggly in the center.
Let it cool completely in the pan, then slice and serve.