Preheat the oven to fan 140C/ conventional 160C/gas 3. Butter and line the base of a 20cm round cake tin. Grind the hazelnuts in a food processor until fairly fine. Add the flour and mix briefly. Add the butter and pulse until it forms crumbs. Add the sugar and eggs and mix briefly. Peel, core and chop two of the pears. Stir the pears and chocolate lightly into the cake mixture.
Spoon the mixture into the prepared tin and smooth the top. Peel, core and slice the remaining pears and scatter over the top of the cake. Press down lightly and bake for 50-60 minutes, until firm to the touch. Cool in the tin for 10 minutes, then turn out and cool on wire rack. Warm the jam and brush over the top. Serve warm or cold.