Instructions

1. Use cake that is a day old and completely cold. Fresh sponge squashes under the knife and the cut faces close up, which slows everything that follows.
2. Cut it into 2.5 cm cubes with a serrated knife and a sawing action, trimming off the browned crust top and sides. The crust dries at a different rate from the crumb and ends up hard where the crumb is light.
3. Spread the cubes on a tray without letting them touch and dehydrate at 52 °C for 6 hours. Turn them over once in the middle of the run so the face sitting on the mesh gets its turn in the air.
4. A finished cube is light for its size and crunches all the way through when you break one open. A cooled cube with any give at the centre is one that will go soft inside its chocolate shell within days, so give the batch more time rather than accepting it.
5. Chill the cookie sheet in the freezer while the chocolate melts. A cold sheet sets the base of each chocolate fast and stops it spreading into a foot.
6. Melt the wafers slowly in a slow cooker on low, over simmering water, or in short bursts in the microwave, and keep every drop of water and steam away from the bowl. Chocolate seizes into a grainy paste at the first splash and there is no recovering it.
7. Spear each cube on a skewer, dip it, and let the excess run back before setting it on the chilled sheet. Scatter your dried fruit, herbs, flowers or coconut on while the surface is still tacky, because nothing sticks to set chocolate.
8. Once they are hard, sit each one in a paper cup and box them. In a cool, dry cupboard they hold two months; the shell is what keeps them, so a cracked or thinly covered cube should be eaten first.