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Not sure which recipe, but it should be fine.
I believe you're talking about powdered sugar when you say 'sugarfine'. If you have regular sugar you can cynically make your own by putting a cup of regular sugar in a blender with a tablespoon of cornstarch and blend until the proper consistency. You may no need to go to the trouble for a lemon tart though. I have certainly made many tarts, pastry creams, and other various pie fillings without confectionary sugar. Generally, you just want to make sure the sugar crystals are well-dissolved whether that be a result of dissolving the sugar in the gelatin mixture, going for the ribbon effect when you combine with eggs, or just 'creaming' the butter and sugar together-- it just depends on the nature of your recipe as to which way you go about it. I would use bout 1.5 times more granulated sugar (by volume) then powdered sugar in terms of substitution.
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No, sugarfine (most likely meaning "superfine") is also known as caster sugar, which is not the same as powdered (aka icing) sugar. It's just a form of sugar that's nicer for cooking with because the crystals are smaller and so they dissolves that much more easily.
You should be fine with regular sugar instead of superfine sugar.
Agree with Sarah R. While probably not necessary, you can make "regular" granulated sugar into superfine by pulsing in the food processor. Not so far that you get powder, and don't add any cornstarch or anything of that nature. You are simply using the power of the food processor to make your sugar granules "super fine".
Cornstarch is non-essential. I only add it to reduce clumping which is nice if there's excess and you end up storing it for any length of time.
I believe you can use regular sugar. The only purpose of superfine is so your filling doesn't have a gritty texture. And if you find that's the case. Just add a little more lemon juice and or cornstarch.
FWIW to both FutureChef and Liz, I go out of my way to avoid cornstarch due to the extremely high likelihood that -- unless organic -- it is GMO. If you must use a thickener/clumping reducer, use Tapioca Starch. If you buy "regular" confectioners sugar, you will find the ingredients include corn starch to prevent clumping; organic confectioners sugar usually uses Tapioca Starch. Enough said. I'm now stepping down from my soapbox...
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