Preheat oven 160°C fan forced. Invert base of a 22cm (base) springform pan. Grease and line base and side with baking paper.
Place chocolate and oil into a heatproof bowl over a saucepan of simmering water. Stir until smooth. Add cocoa, whisk until smooth. Remove from the heat.
Beat eggs and sugar in an electric mixer on high speed for 5 minutes until pale and creamy. Gently fold in chocolate mixture. Add ground hazelnuts and flour, gently fold until combined. Pour into pan.
Top with pears then sprinkle over roasted hazelnuts. Bake 1 hour 15 minutes until a skewer inserted into centre comes out clean. Cool in the pan for 1 hour. Transfer to a wire rack to cool. Dust with icing sugar. Serve with cream.