Sift the flour, cocoa and sugar into a bowl and mix. Beat in the eggs, add the kefir and melted butter, and mix well.

Add the vanilla extract and the baking soda neutralized with vinegar or lemon juice, and mix.

Line the bottom of a 22-24 cm pan with parchment, grease the sides and pour in the batter. Bake at 170°C until a skewer comes out dry, about 45 minutes. Let the cake layer cool.

Using a knife and a 16 cm plate as a guide, cut a circle out of the cake layer; keep the trimmings for the frosting and coating.

For the frosting, beat the softened butter, gradually adding the boiled condensed milk 1 tbsp at a time, until you get a smooth, fluffy frosting.

Crumble half the trimmings in a blender and mix them into most of the frosting.

Cut the 16 cm cake layer into two. Place the first half in the ring, top with the crumb frosting, cover with the second half and press down. Coat the top with some of the plain frosting and refrigerate for 30 minutes.

Remove the ring, coat the sides with the remaining frosting, crumble the second half of the trimmings and coat the sides of the cake with the crumbs.

Refrigerate the cake for another hour. Enjoy!