Set up your barbecue for indirect cooking. Please refer to your handbook for further instructions. Preheat your barbecue for indirect baking.
Using an electric mixer, beat the butter and icing sugar until light and fluffy. Add the flour and custard powder, stir in to combine.
Roll teaspoons full of dough into balls. Place onto two lined baking trays, approximately 3cm apart.
Dust a fork with icing sugar, press down gently onto each dough ball with the fork to flatten slightly.
Once the barbecue has preheated, make adjustments to the burner control(s) if required. Place one tray of the biscuits on and cook for 12-14 minutes, or until just cooked through. Cook the second tray of biscuits. Set aside to cool for 30 minutes.
While the biscuits are cooling, make the lemon filling. Beat the butter and icing sugar with electric beaters until light and fluffy. Add the lemon juice and lemon rind, combine. Once the biscuits have cooled, spread the icing on the flat side on half of the biscuits, then sandwich with the remaining biscuits.