Preheat the oven to 160C/320F/gas 3. Line 2 baking sheets with non-stick baking parchment.
For the meringue, whisk the egg whites in a bowl until soft peaks form when the whisk is removed. Whisk in half of the sugar, a spoonful at a time, whisking well between each addition until the mixture forms a smooth glossy meringue. Sift the remaining sugar, almonds and cornflour into the mixture and gently fold in until just combined.
Spread the meringue mixture in a thin layer on one of the baking sheets and bake for 10 minutes, or beginning to firm up. Remove from the oven. Using a 8cm/3in square cutter, stamp out squares of the meringue and transfer to the second baking sheet. Return to the oven and bake for 10-15 minutes, or until dry and crisp.
For the filling, whisk the double cream in a bowl until soft peaks form when the whisk is removed. Fold in the chestnut purée, followed by icing sugar, to taste. Chill in the fridge until needed.
Press a quarter of the blackberries through a sieve into a small pan. Stir in the sugar and dissolve in the juice over a low heat.
Meanwhile, heat the coffee in a pan and add the pear batons. Poach for 6-8 minutes, or until al dente.
To serve, spread the chestnut filling over half of the meringue squares and sandwich with a second meringue. Sprinkle the sliced marrons glace over. Arrange a stack of pear batons alongside, scattering the remaining whole blackberries around. Drizzle the plate with the sweetened blackberry juice.
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