Although flourless, fudgy, and somewhat dense, this cake doesn’t sink in your stomach. With its shiny, thin chocolate glaze, it’s an attractive dessert for special holidays. If you’re preparing this cake for Passover, be sure to follow your own dietary restrictions.
Special equipment: If you don’t have a 9-inch round springform pan, you can use an 8-inch square pan.
Game plan: The eggs are easier to deal with cold, so separate them straight from the refrigerator, then let them come to room temperature. And make sure there are no traces of yolk in the whites, or they will not whip properly.
Heat the oven to 350°F and arrange a rack in the middle. Coat the bottom of a 9-inch round springform pan with butter or margarine. Line the bottom of the pan with parchment or waxed paper and coat the top of the paper with butter or margarine; set aside.
Place 1/2 cup of the sugar and the water in a medium saucepan and stir to combine. Bring to a simmer over medium heat, stirring occasionally, until all of the sugar has dissolved, about 5 minutes. Remove the pan from heat, add the chocolate, and stir until melted and smooth, about 2 minutes. Set aside to cool.
Place the egg yolks in a large bowl and whisk vigorously until light in color and thickened, about 4 minutes; set aside.
Place the walnuts, remaining 1/4 cup plus 2 tablespoons sugar, matzo meal, and salt in a food processor fitted with a blade attachment. Pulse until the mixture resembles wet sand, about 15 to 20 (1-second) pulses (do not overprocess). Add the walnut mixture to the egg yolks and stir to combine. Add the cooled chocolate mixture to the walnut-egg mixture and stir to evenly combine.
Place the egg whites in the very clean bowl of a stand mixer fitted with a very clean whisk attachment. Whisk the whites on high speed until they hold stiff peaks, about 2 to 3 minutes. Stir about a quarter of the egg whites into the chocolate mixture to lighten the texture. Add the remaining egg whites and gently fold until thoroughly incorporated with no streaks of white visible (make sure not to deflate the egg whites, or the cake will be too thin and dense). Pour the batter into the prepared pan and bake until puffed and set but still a little gooey in the center, about 30 to 40 minutes. (A wooden toothpick inserted 1 inch from the edge should come out clean.)
Remove the pan from the oven and let cool on a wire rack. When the cake is completely cool, unmold it by running a thin-bladed knife around the edge and releasing the springform ring; invert onto a cake plate or serving platter. Peel off and discard the parchment paper.
For the glaze:
Heat the butter or margarine in a small saucepan over low heat until about half of it is melted. Gradually whisk in the chocolate and continue whisking until the chocolate is completely melted and smooth. Remove from heat and let the mixture cool until slightly thickened, about 5 minutes.
Meanwhile, tuck long strips of parchment paper, waxed paper, or foil under the edges of the cake on the plate or platter. When the glaze is ready, pour it over the top of the cake, letting it drip down the sides. Using an offset spatula or knife, spread the glaze evenly over the top and sides of the cake. Gently tap the cake plate or platter to even out the glaze, then pull out and discard the paper or foil strips; the plate will be clean and ready for serving. If you choose, garnish with a few walnut halves arranged in the center of the cake. Refrigerate the cake for about an hour to allow the glaze to set.
Bring the cake to room temperature and serve with whipped cream or vanilla ice cream, if desired.