- Place a cooling rack over a baking tray lined with baking paper.
- Using a long, serrated knife, trim edges of cake off then cut remaining block into 4cm cubes.
- Sift the icing sugar into a large bowl and add butter and lemon juice. Gradually whisk in 1/3 cup boiling water and mix until butter melts. Stir until icing is smooth, then add a little extra water to achieve a thin pouring consistency.
- Place half the 100s and 1000s in a shallow bowl. Using two forks, dip a piece of cake in the icing and turn it to coat well. Transfer cake to the bowl of sprinkles and roll gently to coat on all sides. Place on wire rack to set.
- Repeat with remaining sponge cubes, adding remaining 100s and 1000s to the bwl when necessary.