Begin by crushing the sponge cake into fine crumbs inside a mixing bowl.
Combine the cake crumbs with icing sugar and mix until they are thoroughly blended.
Form the mixture into small, bite-sized balls and arrange them on a baking sheet lined with parchment paper.
In a heatproof container, break the dark and milk chocolate into small pieces.
Melt the chocolate by gently placing it over a simmering water bath (using a double boiler), stirring until the chocolate becomes smooth and completely liquefied.
Dip each cake ball into the melted chocolate, ensuring it is fully coated, using either a fork or skewer. Allow any excess chocolate to drip back into the container.
Place the chocolate-coated cake balls back on the parchment paper.
Optionally, sprinkle your choice of decorations, such as sprinkles or crushed nuts, onto the cake balls while the chocolate is still wet.
Let the chocolate-covered cake balls cool and solidify for a few minutes.
Firm up the cake balls by refrigerating them for about an hour.
Once the chocolate has fully set, your delectable chocolate-covered cake balls are ready to be savoured.