Preheat the oven to 180C/350F/gas mark 4. Grease a deep 20cm/8in round cake tin and line with baking parchment.
Cream together the butter and sugar with an electric mixer until pale and fluffy. Beat in the eggs one at a time, beating well after each addition, add a spoonful of the flour if it looks like the mixture is going to split.
In a separate bowl, sift together the flour, baking powder, cinnamon and salt, add this to the butter and egg mixture and mix in on a low speed. Fold in the chocolate, hazelnuts and pear.
Scrape the batter into the prepared tin and bake for 40-50 minutes until the cake is firm and a skewer inserted into the centre comes out clean. Leave to cool in the tin for 15 minutes or so then turn out onto a wire rack to cool completely.
To make the ganache, place the chocolate in a heatproof bowl. Heat the cream until just boiling then pour it over the chocolate, leave for a couple of minutes then stir until smooth. Pour the ganache over the cake, allowing it to drip over the sides, top with chopped hazelnuts and allow to set before serving.