Preheat the oven to 180℃ (160℃ fan)/350°F/gas mark 4. Grease and line the base of a 20cm (8in) deep round cake tin.
Place 50g (2oz)chocolate broken into pieces in a bowl and stand over a saucepan of simmering water. Make sure the base of the bowl does not touch the water. Stir until chocolate has melted. Remove from the heat and set aside.
Beat 250g (9oz) butter and 300g (11oz) caster sugar together for several minutes until very light and fluffy. Beat in 3 large eggs one at a time.
Sift 300g (11oz) flour and 1½tsp bicarbonate soda into the bowl and fold in with a large metal spoon or spatula.
When the flour is almost folded in, add 250ml (9floz) kefir and continue to fold in until all the ingredients are just combined.
Spoon half the cake mixture into another bowl and mix 1 tsp vanilla extract into one portion and the melted chocolate and 1 tbsp cocoa powder into the other.
Spoon the alternate cake mixture into the prepared tin then marble together with a dinner knife. Bake for 1 hour or until a skewer inserted into the centre comes out clean.
Allow the cake to cool in the tin for 5 minutes before transferring to a wire rack to cool.