This is a really good cake recipe, and while the original I found did use butter, I substituted soy margarine and it worked just as well.
6 eggs separated into yolks and whites, half a cup and 2 tablespoons of sugar, 2 cups chocolate chips, half a cup of soy margarine
Melt the chocolate and the margarine together over a double boiler, being careful not to burn it. Lightly whisk together the yolks and the half cup of sugar, and then fold into the melted chocolate. By hand or with a mixing machine, whisk the egg whites and the 2 tablespoons of sugar together until it becomes a marshmallow-y white color and stays in peaks when the whisk is drawn out of it. Fold this mixture into the chocolate mixture. In an 8 inch round pan, cut a piece of parchment paper the size of the bottom. Grease the paper and the sides and bottom of the pan, then place the paper into the bottom of the pan and pour the cake mixture into it. Bake at 350 degrees for between 20-30 minutes, or until a knife inserted comes out with wet crumbs.