Great question — answering as the UphillTech team:
That's just sugar that has settled on the top – I would either add more drizzle, or make it thicker (less lemon juice compared to the sugar) so it doesn't all penetrate. You could also add it on the top after the cake has cooled if you want, or use icing sugar instead of caster to make more of an icing.
Full answer by dbmag9: https://cooking.stackexchange.com/a/136764 (CC BY-SA 4.0).
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