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.

Photograph by Jonny Valiant