1/2 cup (1 stick) unsalted Passover margarine
1/3 cup plus 2 tablespoons granulated sugar
6 large eggs, separated
1 1/2 cups cooked and mashed sweet potatoes, fresh or canned
1 teaspoon Passover rum extract (optional)
10 ounces good-quality semi-sweet chocolate, melted and cooled
1/4 teaspoon salt
Chocolate Ganache Glaze:
1/2 cup water
6 ounces semi-sweet chocolate, coarsely chopped
Unsweetened cocoa powder, sifted Curls of semi-sweet chocolate Pureed strawberries or raspberries
Preheat the oven to 350°F. Line a 9-inch springform pan with baking parchment.
In a mixing bowl, cream the unsalted margarine or butter with the 1/3 cup sugar. Blend in the egg yolks, then the mashed sweet potatoes, rum extract (if using), and cooled chocolate.
In another bowl, with clean beaters, whip the egg whites gently until they are a bit foamy. Then add in the salt and whip on a higher speed, slowly dusting in the two tablespoons of sugar to form stiff, glossy (but not dry) peaks. Fold one third of the egg whites into the sweet potato/chocolate mixture and work them in well to loosen the batter. Then, gently fold in the remaining egg whites, blending well but taking care not to deflate the mixture. Spoon the batter into the prepared pan and bake for about 40 minutes. The cake rises and looks dry, and slightly cracked on top when done. The middle should be soft but firm. Cool in the pan for 20 minutes, then remove to a wire rack. At this point, the cake can be frozen for up to a month. Even if serving it the same day, chill the cake for an hour or two before finishing it with the ganache glaze.
Chocolate Ganache Glaze:
In a double boiler, bring the water to a gentle boil and add the chopped chocolate all at once. Remove from the heat and stir briskly with a wire whisk until all the chocolate melts and you have a thick glaze or sauce-like topping. Refrigerate for an hour or so. (You can also make this ahead and refrigerate it for up to a week or two. Simply warm it to the right temperature for glazing the cake.)
Invert the cake onto a cardboard circle or cake board so that the smooth, flat bottom faces up. Do not be dismayed if this is not a high cake — it is a torte and is meant to be a little less than statuesque. Pour the glaze over the cake and, using a metal spatula, even out the glaze and spread it along the sides.
Instead of the glaze, you can also simply sift some cocoa over the top of the cake or decorate it with curls of chocolate (using a vegetable peeler and a warmish chocolate bar). The cake can also be offered with a pureed raspberry or strawberry sauce, garnished with chocolate shavings, or left as is, with a citrus leaf, a sweetheart rose, or several berries in the center.