How to Make It
Make the meringue Preheat the oven to 400°. Grease a 9-by-13-inch metal baking pan with baking spray. Line the pan with parchment paper, leaving 1 inch of overhang on the short sides. Grease the parchment with baking spray and dust with the confectioners’ sugar.
In the bowl of a stand mixer fitted with the whisk, beat the egg whites with the cream of tartar at medium speed until frothy, about 2 minutes. With the machine on, slowly beat in the granulated sugar, 1 tablespoon at a time. Increase the speed to medium-high and beat until the meringue is shiny and tripled in volume and stiff peaks form, about 10 minutes. In a small bowl, mix the cinnamon with the cornstarch, nutmeg and salt. Sift the cinnamon mixture over the meringue, then add the orange zest. Fold until well combined. Spread the meringue evenly in the prepared baking pan.
Make the cocoa swirl In a small bowl, combine the espresso powder, cocoa and vanilla with 2 teaspoons of water and mix well. Spoon small dots of the cocoa mixture all over the top of the meringue. Using a skewer or a bread knife, swirl the dots decoratively. Reduce the oven temperature to 350° and bake the meringue for about 25 minutes, until puffed and cooked through. Transfer to a rack to cool completely.
Meanwhile, make the walnut praline Line a baking sheet with parchment paper. Spread the walnuts in a pie plate and toast in the oven for about 10 minutes, until golden. Let cool, then chop into 1/2-inch pieces.
In a small saucepan, combine the granulated sugar, corn syrup and 2 tablespoons of water and bring to a simmer, brushing down the side of the pan with a wet pastry brush. Cook over moderately low heat, swirling the pan, until an amber caramel forms, about 10 minutes. Stir in the walnuts, then scrape the praline onto the prepared baking sheet and spread in an even layer. Let cool completely, then chop into 1/2-inch pieces.
Make the whipped cream In a medium bowl, using a hand mixer, beat the cream with the vanilla at medium speed until medium peaks form.
Assemble the roulade Gently invert the meringue onto a parchment paper–lined work surface with the long side facing you and peel the parchment off the meringue. Spread the whipped cream all over the meringue, leaving a 1-inch border. Sprinkle the walnut praline over the whipped cream, then scatter the grated chocolate on top. Starting at the long side, roll up the meringue to form a log. Transfer to a platter and refrigerate until chilled, at least 3 hours or up to 1 day. Serve garnished with blackberries and pomegranate seeds, if desired.