What kind of sugar works best? Can I use turbinado sugar or brown sugar? Is using only 1 cup of sugar going to alter the cake significantly?

Thanks for the responses! I baked this a few days back for my mum's birthday using about 1 and 1/4 cup turbinado sugar. It turned out moist even without the extra yolk. Delicious - and would have even worked with 1 cup sugar I think.

For the moisture of the cake, I did make the cake last week and accidentally forgot to add half the sugar(I used Japanese brown sugar, btw)required, and the result was VERY moist, I mean much more moist than normal tea cakes. Contrary, the taste was not at all sweet, so I made rich chocolate icing to add. It was good cake.

One commenter on the recipe noted that she halved the sugar and that the cake came out fine and still super moist. Normally I would agree with Suzanne, but it sounds like this cake is just so moist anyway that it might be okay to decrease it here.

You can substitute turbinado sugar for refined white sugar the ratio is exactly the same. If you reduce the sugar you can end up with a drier cake, if you want to reduce the sugar Shirley Corriher suggests adding an extra egg yolk, this will add moisture to the cake. Without adding extra moisture it could alter the cake significantly.