One bite and you'll know why these cake squares, which are kosher for Passover, are such crowd-pleasers. The dessert's caramel glaze and nut topping make them truly special.
Preheat the oven to 350 degrees. Line a rimmed baking sheet with parchment, and spread hazelnuts in a single layer on top. Toast until golden and aromatic, 8 to 12 minutes. Shake the pan halfway through baking to make sure the nuts toast evenly. Transfer to a wire rack to cool. Place 1 1/2 cups hazelnuts in the bowl of a food processor, and pulse until finely ground; reserve. Roughly chop the remaining 1/2 cup nuts, and set aside.
Grease a 9-by-9-inch cake pan with margarine, and line with parchment paper. Grease the parchment, and dust with matzo meal. Whisk together the ground hazelnuts and 1/2 cup matzo meal, and set aside.
In the detached bowl of an electric mixer, whisk egg yolks, whole eggs, and 1 cup sugar over a pan of simmering water until warm to the touch. Transfer mixture to the bowl of an electric mixer fitted with the whisk attachment. Beat on medium-high speed until mixture is thick, light in color, and leaves a ribbonlike trail on the surface for 3 seconds when you raise the whisk. Fold in matzo-meal mixture. Fold in melted margarine.
Transfer batter to prepared pan; smooth top with an offset spatula. Bake until a cake tester inserted into the middle comes out clean, about 40 minutes. Transfer to a wire rack to cool completely, about 40 minutes.
Invert pan, and turn out cake on wire rack so bottom of cake faces up. Set rack on a rimmed baking sheet.
Spread remaining 1 cup sugar on the bottom of a 10 1/2-inch skillet. In a small bowl, whisk together orange juice and potato starch; set aside. Place skillet over medium-high heat. Without stirring, let sugar begin to melt. When sugar begins to turn golden around edges of pan, about 4 minutes, reduce heat to medium; stir melted and unmelted sugar together. Continue stirring until all sugar has melted and is clear and golden, about 1 minute. Stir in orange-juice mixture; cook, stirring, 1 minute.
Remove skillet from heat, and stir in the chopped hazelnuts. Pour the glaze over top of cake, and spread the nuts evenly with the spatula. Let cool. Cut into 3-inch squares, and serve.
You may substitute almonds or pistachios for the hazelnuts.