Preheat oven to 180°C. Line two baking trays with baking paper.
Sift self-raising flour and custard powder together. Beat butter and sugar together with electric beaters until pale and creamy. Add the egg and beat until just combined. Stir in flour mixture to form a soft dough.
Roll tablespoonfuls of the mixture into balls. Place onto trays, allowing room for spreading. Flatten slightly using fingertips.
Bake for 15 minutes or until lightly golden. Remove from oven and cool completely. Gradually add 2-4 tsp water to icing sugar and mix to a thick pourable icing. Spread a little onto each biscuit. Sit for 10 seconds then top with a freckle or sprinkles.
Freckle chocolates can be substituted with 2 tbsp chocolate sprinkles.