Beat the sugar and eggs with a mixer. Add the yogurt, oil, baking soda and vanilla. Mix again.
Pour in the flour and continue stirring. Set aside 1 cup from the mixture and beat it with the cocoa.
In the mixture without the cocoa, add the crumbled chocolate and stir. Grease a cake form and pour in this mixture.
Then add the mixture with the cocoa. Heat the oven on high and you put the cake inside, then reduce it to moderate heat. Bake until ready.