2) Put the chocolate in a heatproof bowl. Bring a saucepan filled with 3cm or so of water to a very slow simmer; place the bowl of chocolate onto the pan (without touching the water) and stir the chocolate occasionally until it is melted and smooth. Alternatively, put the chocolate in microwave safe bowl. Microwave on half power until soft, about 1 minute, depending upon the power of your machine. Remove and allow to cool.
3) Sift the flour, baking powder and salt into a small bowl.
4) Cream the butter and sugar in a medium sized bowl with an electric blender on medium-high speed for about 4 minutes, until the mixture is light and fluffy. Add the eggs, 1 at a time, beating well. Stir in the chocolate, orange peel, and vanilla and mix until fully combined. Add a third of the flour mix and whilst stirring contantly, add a third of the milk. Continue adding the flour and milk until you have a thick batter.
5) Pour the batter into the prepared cake tin. Bake in the centre of the oven for about 30 minutes or until a skewer comes out clean.
6) Cool the cake in the tin for 10 minutes, then turn out onto a rack and allow to cool completely.
7) Serve dusted with icing sugar and with whipped cream or ice cream, if desired.