Preheat oven to 350 degrees F. Butter and flour an 8-inch cake pan.
Put the chocolate in a heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl on the pan (without touching the water). Stir occasionally until 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. Cool.
Whisk the flour, baking powder and salt in a bowl.
Beat the butter and sugar in a medium bowl with an electric mixer on medium-high speed until light and fluffy, about 4 minutes. Add the eggs, 1 at a time, beating well after each addition. Stir in chocolate, orange peel, and vanilla and mix until fully combined. Add in flour, salt, and baking powder in 3 parts alternating with the milk to make a thick batter.
Pour batter into prepared pan. Bake in the middle of the oven until a tester comes out clean, about 30 minutes.
Cool cake in the pan for 10 minutes, then turn out onto rack to cool completely.
Serve dusted with confectioners' sugar and with whipped cream or ice cream, if desired.
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