Preheat the oven to 355F (180C).
In a medium bowl, mix butter, sugar, and vanilla extract for about 1 minute. Add eggs and continue mixing until eggs are well incorporated.
Add kefir (or buttermilk) and continue mixing for a little bit more.
Add flour and baking powder and mix until you get a consistent batter without lumps. The batter should have the consistency of pancake batter. Start with 1.5 cups (200 g) of flour first and then add more, one tablespoon at a time, if needed. I usually don't have to.
Divide the batter into two equal parts. Add cocoa powder to one half and mix.
Next, add about 1/3 of the plain batter to the greased cake pan and spread across the bottom, followed by 1/3 of the cocoa batter. Continue layering until you've used up all batter. You should end up with 3 plain and 3 cocoa batter layers. Smooth out the top and scrape down the sides of the cake pan.
Bake for about 50-55 minutes or until a wooden stick comes out mostly clean. Do not over-bake! Start checking early, after about 45 minutes.
Pull the cake out of the oven and let cool on a cooling rack for 20 minutes.
Once the cake has cooled down, quickly prepare the glaze. In a medium saucepan, mix milk, butter, sugar, chocolate, and cocoa powder. Slowly heat the mixture over very low heat, constantly stirring, letting the chocolate and the butter fully melt, and until the chocolate mixture is silky smooth. That should take about a minute or two. Set aside and let cool slightly.
Spread the glaze over the cooled-down cake and let it set for about 10 minutes before slicing the cake and serving.