Preheat the oven to 200ºC/400ºF/Gas mark 6

On a baking sheet, bake the sweet potatoes whole in their skins until soft (about half an hour, turning the potatoes over halfway through)

Slice the potatoes in half, scoop out the flesh and discard the skins

Put the purée into a large saucepan, add the sugar and heat to boiling for about 5 minutes, stirring all the time to prevent it sticking to the bottom and burning. This is to get rid of some of the liquid

Take it off the heat and allow to cool for 10 minutes

Add the 3 eggs to the potato & sugar mix and combine well

In a large-sized bowl, add all the remaining dry ingredients - ground almonds, cornmeal, plain flour, desiccated coconut and chopped crystallised ginger - and combine well

Add the dry ingredients to the wet potato mixture in 3 stages, mixing with a wooden spoon after each addition

Tip the mixture back into the large bowl, cover in cling film and refrigerate for a couple of hours or overnight

Preheat the oven to 220ºC/425ºF/Gas mark 7 (200ºC if fan-assisted)

Grease 2 large baking trays well

With floured hands, make small lozenge shapes, each using about a tablespoonful of mixture and lay on the baking tray

Brush each with the beaten egg yolk

Bake for 10-15 minutes until lightly browned

As this batch is cooking in the oven, prepare the next batch for the other baking tray

Remove from the tray and allow to cool on a wire rack