Preheat your oven to 325°F (160°C) and prepare two tall 8” or 9” round cake pans by greasing them with butter or cooking spray and dusting with flour.

In a large mixing bowl, cream together the softened unsalted butter and granulated sugar on medium speed until light and fluffy, about 4-5 minutes.

Add the eggs one at a time, mixing well after each addition, then add the vanilla extract and fine sea salt, blending until smooth.

In a separate bowl, sift together the cake flour, black cocoa, and baking soda. Gradually mix this dry blend into the wet batter, alternating with kefir or buttermilk.

Divide the batter evenly between the prepared cake pans and smooth the tops. Bake for 40-45 minutes until a toothpick inserted comes out clean.

Let the cakes cool in the pans for 5-10 minutes before transferring them to wire racks to cool completely.

For the buttercream, beat the softened unsalted butter until creamy, then add black cocoa and powdered sugar, mixing until smooth. Optionally mix in melted semi-sweet chocolate.

Once the cakes are completely cool, frost between the layers and on the top and sides of the cake with the black cocoa buttercream.