Sieve together refined flour and baking powder. Chop walnuts. Grease a microwave dish.
Take grated chocolate and butter in a microwave bowl and melt in a microwave oven on MEDIUM (70%) for one minute.
Stir the mixture and add castor sugar and stir again to mix well. Add the sieved flour and mix well.
Add milk and mix to get a pouring consistency. Add chopped walnuts and vanilla essence. Pour the batter into the greased microwave dish and level the surface.
Bake in the microwave oven at HIGH (100%) for five minutes. Check for doneness. If not done bake for one more minute at MEDIUM (70%).
If baking in a convection oven preheat the oven to 180°C and then bake the cake for twenty five to thirty minutes at 180°C.