Combine the flour and baking powder in a bowl. In another bowl, mix together the butter, caster sugar, egg, vanilla extract and cocoa powder with an electric mixer. Gradually add the flour mix until incorporated.
Bring the dough together with your hands, then place on top of clingfilm and flatten into a disk shape. Wrap in the clingfilm and chill for 1 hour until firm.
Preheat the oven to 180°C/gas mark 4. Roll out the chilled dough between two sheets of baking paper, then cut out the biscuits using a 6cm cutter.
Place the biscuits on a large, lined baking tray and bake in the oven for 8-10 minutes.
Remove the biscuits from the oven. While they're still warm, cut out a little hole on the top of each one. You can use a 1cm round piping nozzle to do this.
Leave to cool on the baking sheet for 5 minutes, then place on a cooling rack and leave to cool completely. Shake over the lustre powder to give the biscuits a gold shimmer.
To decorate, mix the icing sugar with 1 tbsp cold water. Spoon into a piping bag and ice the biscuits.