Mix together the all-purpose flour, baking powder, baking soda and keep aside.
Beat together the eggs and sugar until it turns pale and smooth.
Now add the yogurt and oil. Beat everything again for a while. Add the flours slowly little by little.
Stir everything well but do not over mix.
Meanwhile melt the chocolate bar in a microwave bowl with a little water in the microwave oven for 4 minutes. Slowly pour in the melted chocolate to the wet ingredients and mix for a while. Gradually add the dry ingredients and mix until everything gets well mixed.
Line a baking sheet in a greased vessel. Pour this cake mixture.
Meanwhile, heat two cups of water in pressure cooker.
Arrange the vessel; close the cooker with a lid, steam cook on high heat for 5minutes.
Put the flame on simmer and cook for 40 minutes (check the water level in between)