Pressure cook the soaked Rajma until it is cooked well. Drain the water completely and keep aside.
Chop the chocolate and melt the chocolate in a sauce pan or you can use double boiler method.
In a grinder/blender jar, add all the ingredients and grind them smoothly.
Preheat oven @180° for 10 mins. Grease the square cake pan with oil and dust it with cocoa powder.
Pour the grinded mixture into the pan and tap 2-3 times. Garinish with chopped walnuts.
Bake at preheated oven @180° for 30 mins. Remove from oven and let it cool down completely.
Demould, cut and serve.