Preheat the oven to 375 F. Prepare two muffin tins with liners.
Beat the butter and sugar together for about 5 minutes, until the mix is light and fluffy. Add the eggs and beat for a few minutes more. While the mixer is running, add the baking soda, vanilla and salt.
Into the bowl sift the black cocoa and cake flour (if they are lumpy). With the mixer on, slowly pour in the kefir. Beat just until you have a smooth batter.
Fill muffin liners halfway with batter. Bake for 15-20 minutes, until the tops spring back and are firm.
For the buttercream:
The butter and cream cheese should be soft and at room temperature before you start. Beat them together until no lumps remain. Sift in the powdered sugar and black cocoa. Add the salt & vanilla and beat until it’s smooth.
Assembly & storage:
When cupcakes are completely cool, pipe or spread buttercream on top. Store in an airtight container. Cupcakes are good for a few days.