Preheat the oven to 170C/350F/Gas 4.
Beat the butter until very, very soft (preferably using an electric stand mixer) and there are no lumps, then beat in 100g/3½oz icing sugar for a minute.
Sift the ground almonds and remaining icing sugar together over the mixture and stir in.
Continue mixing, adding the egg whites one at a time.
Then mix in the flour.
Add the orange zest and orange extract.
Leave the mixture to rest for 20 minutes.
Meanwhile, mark out 7cm/2¾in lines, set at least 3cm/1¼in apart, on baking parchment, to guide where to pipe the biscuits. Flip the paper over and pipe on the unmarked side (the lines will show through).
Fill a piping bag fitted with 1cm/½in plain nozzle with the biscuit mixture and pipe out. Smooth the tips down at the end of the line with a finger wetted in water.
Bake for 8-9 minutes until the pale biscuits are just colouring and the centre is just golden-brown, with the edges slightly darker. This may take up to 10-12 minutes.
Remove the biscuits from the oven and leave on the tray for 1-2 minutes. Using a palette knife transfer the biscuits to a wire rack to cool. Alternatively you can leave the biscuits to dry out and cool on the tray if slightly underdone.
To decorate, combine the chopped chocolate and ground cardamom seeds.
Melt two-thirds of the chopped white chocolate very gently in a heat-proof bowl set on top of a pan of simmering water. Heat the chocolate to no more than 45C/113F (use a thermometer to measure this), remove from the heat and then add the remaining white chocolate, stirring continually until the temperature drops to 32C/90F. Transfer to narrow or steep sided small bowl.
Half dip each biscuit in the chocolate and then lay flat on baking parchment to dry.