Grate chocolate. Cut the butter into cubes. Grease a Microwave proof dish. Melt butter and chocolate in a Microwave proof bowl in the Microwave oven for one minute on Microwave HIGH (100%).
In another bowl sieve refined flour and baking powder. Mix the melted butter and chocolate well.
Add powdered sugar, vanilla essence and mix. Add the flour mixture and mix. Correct the consistency by adding milk. Add walnuts and mix gently.
Transfer the mixture into the greased Microwave proof dish. Bake in the Microwave at HIGH (100%) for seven minutes.
Allow standing time of five minutes. Cool, slice and serve.