Similar to a regular risotto process, melt the unsalted butter in a hot pan and add the rice. Stir for about a minute or until the rice is translucent. Pour a cup of milk into the pan and stir until the milk reduces. Add the sugar and stir continuously. As the milk evaporates, add another 1/2 a cup of milk to the rice and continue stirring. Repeat this process until the rice is at the 'al dente' stage.
Add the chocolate to the rice, and stir until the chocolate has melted and the milk has reduced to a creamy consistency.
Serve the dessert immediately in cups and finish with a sprinkle of walnuts on top.