125 gmhazelnut meal80 gmplain flour20 gmDutch-process cocoa, plus extra to serve1 tspbaking powder280 gmdark chocolate (70% cocoa solids), finely chopped200 gmunsalted butter, coarsely chopped25 mlespresso or coffee liqueur5eggs, separated220 gm(1 cup) raw caster sugarTo serve:crème fraîcheRoast rhubarb1 bunchrhubarb, trimmed and cut into 5cm lengths110 gmraw caster sugar2oranges, juice only1vanilla bean, scraped seeds only
Preheat oven to 200C. Place rhubarb in a colander, rinse under running water, combine in a bowl with remaining ingredients, stir to coat. Transfer to a small roasting tray, cover with foil, roast until tender (10-15 minutes), set aside.
Reduce oven to 180C. Combine hazelnut meal, flour, cocoa and baking powder in a bowl, set aside. Melt chocolate, butter and espresso in a heatproof bowl over simmering water, stirring occasionally until smooth (2-3 minutes). Remove from heat, stir in yolks.
Whisk eggwhites in an electric mixer until firm peaks form (2-3 minutes), gradually add sugar, whisking until thick and glossy. Fold into chocolate mixture, then fold in hazelnut mixture. Spoon into a 22cm-diameter buttered and base-lined springform cake tin and bake until a skewer withdraws clean (30-35 minutes). Cool completely in tin, remove, dust with extra cocoa and serve with crème fraîche and roast rhubarb. Store at room temperature. Best eaten on day of making.