Preheat the oven to 190°C (fan oven 170°C), gas mark 5. Invert 6 small dariole moulds on a baking sheet.
Cut each sheet of filo pastry into about 6 rectangles and keep them covered with a tea towel while working. Using half the pastry, drape the rectangles in twisted layers over the moulds, brushing very lightly with melted butter between each one.
Bake in the preheated oven for 9-10 mins until pale golden and crisp. Leave to cool for 10 mins, then ease off the moulds. Repeat with remaining pastry to make 12 tartlet cases.
To make the mousse, chop the chocolate into 50ml (2fl.oz) of the cream in a heatproof bowl and melt slowly over a pan of gently simmering water.
Cut the leaf gelatine into small pieces, put in a bowl with 2 tbsp water and leave to soak for 10 mins. Pour the gelatine and water into the chocolate cream and stir gently until dissolved. Stir in the egg yolk.
In a clean bowl, whisk the egg white until it forms soft peaks, then gradually whisk in all the caster sugar until stiff. In a separate bowl, whisk the remaining cream until it just begins to hold its shape.
Put 1 tsp of conserve into the base of each tartlet case. Using a metal spoon, fold the egg white and whipped cream into the chocolate and spoon immediately on top of the conserve. Chill for at least 30 mins to set.
Stir together the remaining conserve with the citrus juice and gently fold in the raspberries and blueberries. Just before serving, spoon the fruit onto the chocolate mousse, decorate with mint sprigs and a dusting of icing sugar.
To prepare ahead, complete the filled filo tartlets and freeze. Stir the fruit into the conserve about 2 hrs before needed and chill. Thaw the tartlets in the fridge for 20 mins, top with fruit and serve.
If you don't want to make the tartlets, spoon the conserve into small shot glasses, top with the mousse as above and chill to set.
If you don't have dariole moulds to shape the tartlets, use scrunched up balls of kitchen foil.
