Make the cake: Preheat the oven to 350 degrees F. Coat the bottom and sides of a 9-inch-round cake pan with cooking spray and line with parchment paper. Put the walnuts, cinnamon and 1/4 cup sugar in a food processor; pulse until finely ground but not powdery. Whisk the egg yolks, orange zest and vanilla, if using, in a small bowl.
Beat the egg whites and salt in a large bowl with a mixer on medium-high speed until foamy. Beat in the remaining 1/4 cup sugar, 1 tablespoon at a time, until soft peaks form, about 8 minutes. Fold in the yolk mixture and dates, then fold in the ground walnut mixture. Scrape the batter into the prepared pan. Bake until the cake is golden and a toothpick comes out clean, 23 to 25 minutes. Let cool in the pan on a rack, then run a knife along the sides and invert the cake onto a platter.
Make the glaze: Put the chocolate, margarine and honey in a microwave-safe bowl, cover with plastic wrap and microwave until the margarine melts, about 2 minutes. Whisk until smooth. Cool slightly, then pour over the cake. Top with walnuts.
If you are concerned if the oils or other ingredients in these recipes are suitable for Passover, seek non-dairy substitutes or ingredients that are certified kosher for Passover.