Mix all the ingredients for the syrup in a pot. Boil for 7 min. after boiling point and then leave it to cool.
Beat the eggs with the sugar with the mixer on high, add the softened butter, vanillas and pinch of salt. Beat for about 5 min. Add the yoghurt and beat.
Next, add the flour and stir lightly with a spatula. Dissolve the cocoa in the hot water, add 1/4 of the cake mixture and stir until homogeneous.
Make circular motions in the mixture with a spoon. Bake in a preheated 338°F (170 °C) oven for about 50 min.
Pour the cold syrup evenly over the hot cake. Once the cake cools as well, take it out of the form.