Preheat oven to 180C. Using a food processor, process hazelnuts and icing sugar until nuts are finely ground.
Using an electric mixer fitted with a paddle, beat butter, rind, vanilla seeds, hazelnut mixture and a large pinch of salt until creamy, then add flour, stirring until just combined (do not overmix).
Roll half-teaspoon of dough into balls and arrange about 6cm apart on baking paper-lined oven trays. Bake one tray at a time in centre of oven until golden (12-15 minutes), then cool on trays over a wire rack. Repeat with remaining biscuits.
For chocolate filling, combine chocolate, butter and cream in a heatproof bowl over a saucepan of simmering water, stirring occasionally, until melted. Stir until glossy and smooth, then cool to room temperature.
Spread half-teaspoon of chocolate filling onto the flat side of half the biscuits (you may have a little filling left over), then sandwich with remaining biscuits, pressing lightly together to seal. Biscuits will keep in refrigerator in an airtight container for up to 1 week. Bring to room temperature before serving.