Knead together butter, grated marzipan, flour, and custard powder until combined into a dough.
Tear off walnut-sized pieces from the dough and shape each into a ball.
Place the balls on a baking sheet lined with baking paper. Make a small hollow on top of each ball using the handle of a wooden spoon.
Bake in a preheated oven at 160°C (320°F) using fan/convection setting for 10-12 minutes, until light golden. Adjust baking time if your balls are significantly larger or smaller.
Remove from the oven and let cool completely.
Spread chocolate icing around the hollow of each cooled ball.