Melt the butter in a frying pan and stir in the sugar. Cook over a gentle heat until the sugar dissolves. Continue to cook until the mixture starts to caramelize. Remove from the heat.
Peel, core and chop the apples into the pan and cook gently for 3–5 minutes until the apples start to soften. Add the blackberries and cook for a further minute. Stir in the vanilla bean paste and lemon juice. Spoon into the vol au vent cases, packing the mixture down gently. Place on a baking sheet and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 5 minutes to warm through.
Reduce the syrup left in the frying pan for about 5 minutes until thickened, then spoon onto the fruits. Lightly dust with icing sugar and serve decorated with mint sprigs or edible flowers.